Plants and other living things are classified—not in the top secret, CIA-way that something can be classified, but because they are grouped with other closely related plants. These groups show evolutionary relationships between plants, and all the plants in a group share certain characteristics.
You might be familiar with the old saying, King Philip Came Over From German Soil as a way to remember the levels of organization for living things. These arefrom most general to most specific:
It used to be said that there were five kingdoms of life: plants, animals, fungi, monerans and protists. That way of organizing things was a little too focused on the large multicellular organisms, because it turns out that most of the living things on our planet are microorganisms.
Now, we have an extra category at the top, called the three domains of life. These are
Plants (and animals, and fungi and some other organisms) are eukaryotes, so they all fall within the eukaryote domain. When we talk about plant classification, we can still use phylum and family and so on because these are categories within the eukaryote domain.
The first big differences we will go over are some of the different phyla (that's the plural of phylum). There are 12 phyla of plants, but don't worry, you don't need to know them all. We'll meet a few of them later.
Even though class and order come next after phylum, most plant lovers tend to think about families of plants. This is because plants within a family share a lot of similarities, just like people do within a family. You can use plant traits, such as the shapes of flowers and the way the leaves are arranged, to tell what family a plant is in.
A guide that people use to figure out what family (or genus or species) a plant belongs to is called a key. Here's a key you can check out.
The process of organizing plants or other living things into these groups is called taxonomy. The organisms do not have to be currently living, either—taxonomy can be applied to fossils. People have been interested in taxonomy for centuries, but the ability to study and analyze relationships based on DNA makes it easier to know how closely plants are related to each other.
The practice of analyzing DNA to understand relationships between organisms is called phylogenetics. This is usually used for levels above species (genus and above). Even though families of plants share many characteristics, sometimes these traits do not always tell the full story of evolutionary history. After all, a group of plants could lose a trait that the rest of the family has, or could evolve a new trait that no one else in their family has.
The person who came up with the hierarchical classification of plants and the use of a genus + species name to describe plants (which is still used) was Carl Linnaeus, a Swedish botanist in the 1700s. This type of classification is sometimes called Linnaean taxonomy.