There is a $64,000 question. It seems the accepted definition has changed through history.
Long ago it was simple. If it moved around it was an animal (anima = movement) and if not, it was a plant (anchored). So scientist created the subdisciplines of biology called zoology (study of animals) and botany (study of plants). Back in ancient times my introductory biology class was even listed in the university catalog as Bot/Zoo 102!
Of course popular culture decided that "animals" meant only the warm fuzzy or cute ones nursing babies (mammals). More-enlightened people figured out that most of the animals lack backbones and are really cold and prickly or cold and slimy (think invertebrates). In pop culture, plants are not really alive (of course this is wrong!).
As we studied the plants and the animals more closely, scientists decided that our old ideas were really erroneous. There are so many organisms out there and their features intergrade finely. It became hard to distinguish organisms in one way, but clearly the two-kingdom (sorry ladies...it's not my fault) system was inadequate. In particular, the group called plants was really a mixture of very-different organisms; it needed to be divided.
Enter the six-kingdom system. The Bacteria and Archaea are most unique (prokaryotic). The two kingdoms have organisms that lack nuclei or other membrane-bound organelles. Protista is an artificial catch-all with intergrading forms and difficult organisms. Most are aquatic and are fundamentally unicellular. These are crowned by three well-developed kingdoms: Plantae (Plants), Animalia (Animals), and Fungi (Molds etc.). Because zoologists had a very small idea of what should be animals, botany was stuck with organisms that are today found in four of the six kingdoms!
Blue-green "Algae" (= cyanobacteria) in Bacteria
Brown, Red, Green, and many other "Algae" and Water Molds in Protista
Mosses, Ferns, Gymnosperms, Angiosperms in Plantae
Molds, Mildews, Mushrooms, Rusts, and Smuts in Fungi
Obviously, then, this course would have to cover a lot of territory if it were "traditional." Animal courses have the luxury of focusing on one small group. But "modern" botany is more limited than it once was. We still need to cover more than animal courses and thereby get the advantage of seeing a vast array of diversity in form and function. However, I am eliminating all prokaryotes except cyanobacteria (because they are the closest relatives to chloroplasts), all algae except the green algae (because they are the closest relatives to true plants), and all fungi (they are not plants by any modern concept). In this way I have carved huge sections out of what was once botany...leaving cyanobacteria and green algae as the only non-plants in this course.
(excuse me here for focusing on Kingdom Plantae for a moment)
It is important that biology as a discipline develop some orderly way of handling taxonomy, and I am assuming that Biology 122 or 125 has provided you with much information on this subject. Here are the levels of classification that are commonly recognized:
Kingdom (Bacteria, Archaea, Protista, Animalia, Fungi, Plantae)
Division (Anthophyta = Magnoliophyta = flowering plants)
Species Latin binomial (two names)
cultivar (author's language with single quotes)
Why do we use Latin? Well, Latin was selected as the universal language for taxonomy very early on, which means that you can pick up a journal in botany written in just about any language and somewhere in the title or abstract the taxonomic name will appear in Latin. You will instantly know whether or not this paper is working with the taxon you prefer to study. Secondly, Latin is a dead language. Because it is no longer in use by people, the meanings of root words do not change with time. This gives the language some stability that would be demanded by a universal language. Thirdly, because the international bodies that determine nomenclature for taxonomy come to agreement on the names to be used, each organism has a particular name that is unique to that organism.
Common names, while cheerful and sometimes cute, do not share these properties with Latin names. So there are at least 100 unrelated plants called "Black-Eyed Susans" world wide...if you use "Black-Eyed Susan" in your published article without any reference to a Latin binomial, other scientists world-wide would make different assumptions about the plant you worked with, leading to chaos in the literature. So as a developing scientist I hope you will learn to appreciate Latin binomials and USE them.
Humans have been selecting various forms of particular taxa for use as food and so on. This unnatural selection process has produced plants that are unique but are still belonging to a single species. Good examples include the many cultivated subspecies of Brassica oleracea. The names below are all Latin subspecies or variety names that go with certain common vegetables. If you dissect the Latin roots of these names, I think you can see that they are just as descriptive as common names.
capitata = cabbage
acephala = kale
gemmifera = Brussels sprouts
italica = broccoli
botrytis = cauliflower
caulorapa = kohlrabi
Of course human selection created more than just subspecies! Indeed each subspecies includes many cultivated varieties or cultivars of each subspecies. For example, a perusal of any seed catalog will find a section of cabbages that include these two cultivars:
Brassica oleracea capitata 'Late Flat Dutch'
Brassica oleracea capitata 'Copenhagen Market'
You might notice that the cultivar name is NOT Latin and is surrounded by single quotes rather than typed in italic font.
Plant classification is one of the older pastimes in botany. Every culture, society, and religion has taken it as its duty to name and organize the plants in its area into some manageable arrangement. The arrangement at first was meant to facilitate communication between people about the plants under discussion.
The earliest organizations, naturally, were based upon the uses of plants by people. Obviously some plants were useful as vegetables, others were fruits, some make great spices for other foods, some were a good source of sugar, or perhaps starch. Oils could be extracted from others. Some were good for spinning fibers into thread for clothing. Yet others were ornamental.
But as is true of any classification scheme based upon human uses, there are problems of several sorts. The uses of plants by one civilization might not be the same as those by another. Even among individuals in the same society, one person's fruit is another's animal fodder. Moreover there is the conundrum of what to do with plants that present several different uses. George W. Carver demonstrated many uses for peanuts. Henry Ford produced an automobile with body parts made of soybean products. But the worst problem with human-use classification is that it puts related organisms in different classification categories. There is no relationship between the classification system and the evolutionary pathway through which the diversity was obtained.
Other early attempts at classification focused upon the form of the plant. Thus one category would be the herbs; here we use herb in its botanical sense meaning non-woody plants. A second category would be woody plants. These plants have secondary xylem and would need to be subdivided into several sub-categories of form: shrubs, vines, and trees. Each of these categories would have to be subdivided into deciduous and evergreen. Of course whether an evergreen vine is closer related to an evergreen tree or to a deciduous vine is a very good question. And of course there are both woody and herbaceous vines. It is sounding messy. Also, we find that a natural grouping may have members in all of these "unnatural" categories.
Humans have also noticed that the seasonality of plants varies and so can be another way to divide up the plants into categories. Thus in one category we have annual plants which are planted in the spring, flower that summer, and die in the fall. Then we have the perennial plants which are planted in one year, grow vegetatively in the first year, over-winter, and then flower in each year thereafter. Nurseries like to sell annuals because they know customers will come back next year to replace them all. They sell an entire flat of annuals for a small price. Perennials are planted, and then never replaced, so nurseries place a premium price upon them. However, there is a real "gotcha" among the perennials in some nurseries, however...biennials act like perennials in the first and second year, but die all the way to the ground at the end of that second year. You will have to replace those. Many seed catalogs are divided into annual and perennial categories. However even a casual gardener will be able to find plants of the same kind in both the annual and perennial categories. Certain species of a genus may be perennial, and other species of the same genus may be annual. It is not a natural division.
Another way of dividing the plants is obvious in nursery catalogs: by climate. Tropical plants originate in areas of the world between the tropics of Cancer and Capricorn. This part of the world is very warm year-round with frost never occurring in the year. This is explained as the sun passes directly overhead on at least one day each year, and may do so for much of the year. Tropical plants generally cannot withstand any kind of frost. Subtropical plants are slightly more hardy and can tolerate light frost but not heavy freezes. Hardy plants can tolerate frozen periods of various durations and temperatures. The hardy plants are classified on the basis of hardiness zones. In North America these range from Zone 1 (extremely cold and long winters) to Zone 10 (no frost at all). Subtropical plants survive in zones 8-10. Some hardy plants can only survive in the warmer zones (7-10) while others are tough enough to survive in zones 1 and 2! In nursery catalogs the coldest zone for each plant is noted and gardeners are well-advised to heed those. Connecticut is zone 6 except for the northwest corner which is zone 5. Along the shore one might experiment with some zone 7 plants if a sheltered spot is available. However, again, close relatives can be tropical plants and extremely hardy plants. This is not a natural classification scheme. It is useful but not based upon genetics and evolution, the cornerstones of biology!
All of the possible classification criteria above create artificial groups of virtually unrelated plants AND separate out close relatives in what appear to be distinct classification categories. Juniper is an evergreen conifer that is a tree, holly is an evergreen dicot that is a tree. These criteria also artificially separate related plants. Spruce is an evergreen woody tree, larch is a deciduous woody tree but both are conifers. So we really needed something better; this was recognized by Linnaeus and others who established the foundations of Botanical Taxonomy or what is known as Systematics.