Muggles' Guide to Harry Potter/Magic/Herbology

Overview
Herbology is the study of magical plants and how to take care of, utilize and/or combat them. Herbology lessons are shown in each of the first six books.

The subject is depicted as closely related to Potions, due to the nature of the plants, many of which are cultivated for extracting useful ingredients. Throughout the series, Herbology is taught by Pomona Sprout.

Extended Description
Herbology is a core subject; it is one of the seven academic subjects taught in the first year. Harry, who chooses to pursue a career as an Auror, takes Herbology at NEWT-level.

Being located in greenhouses on the grounds, trips to and from Herbology lessons are written to depict the weather at the time (for example, Harry Potter and the Half-Blood Prince, chapter 14). There are at least three greenhouses described in the books, holding a variety of magical plants of varying degrees of lethality, including mandrakes, bubotubers, and Snargaluffs (which are valued for their pods).

Herbology is also, initially, the only subject in which Neville Longbottom excels. Neville will later show aptitude for Charms and Defence Against the Dark Arts.

Analysis
As mentioned, Herbology and Potions are reasonably closely related. We see one interaction between Professor Sprout and Professor Slughorn, where they are discussing the properties of a plant that Slughorn needs for his first-year Potions class. We have to assume there are similar, though possibly not as cordial, interactions earlier between Professor Sprout and Professor Snape.

Like Charms, Herbology is a practical class, with lots of noise and activity going on. As a result, it is a class where Harry, Ron, and Hermione can discuss their plans. It is likely for this reason that we see as much of Herbology as we do, and also possibly why all three of the Trio are able to continue Herbology studies at NEWT level.

Greater Picture
Herbology seems to the uninitiated to be a relatively straightforward and harmless branch of magic, but we are somewhat disabused of that notion by the appearance of the Venomous Tentacula, first mentioned in Harry Potter and the Chamber of Secrets. Professor Sprout's offhanded treatment of the plant in that episode is misleading; much later, we see a Death Eater being reeled in by Tentacula. Many plants are only handled with protective gear, and in the climactic battle at Hogwarts we see many magical plants being used as weapons.

In the Epilogue, it is revealed that Neville has become the Professor of Herbology at Hogwarts.