"Lucky you pay attention in Herbology, Hermione."
-- Ron Weasley after they escaped the Devil's Snare (PS16)
Herbology class at Hogwarts is the study of magical plants and fungi, including their care and their magical properties and uses. Some magical plants form important ingredients in potions, others have magical effects in their own right.
At Hogwarts, all students are required to attempt an O.W.L. in Herbology, so all first through fifth year students take the class, which is taught by Professor Sprout. Herbology classes are held in the greenhouses and include plenty of hands-on activities, including pruning alihotsy bushes, repotting mandrakes, and harvesting bubotuber pus. Students are also assigned essays as homework (OP13).
For the Gryffindors, Herbology is a double class with the Hufflepuffs. Neville Longbottom is particularly good at Herbology. It's usually the only class in which he raises his hand.
Equipment
dragon dung compost
Used for planting (OP13). Sold in Diagon Alley at the apothecary (CS6).
dragon-hide gloves
Often worn when handling dangerous plants such as bubotubers.
ear muffs
Used by students to protect themselves from plants like the Mandrake which can harm with the sounds they make; these ear muffs are apparently magical as they completely block all sound (CS6).
slug pellets
Hagrid has been known to carry a few of these around in his pockets
Flesh-Eating Slug repellent
Occasionally required to keep Flesh Eating Slugs out of Hogwarts' cabbages, these must be bought on Knockturn Alley, which suggests that they are somewhat Dark in nature (CS4)
Mooncalf dung
When collected at the full moon (probably the ONLY time you can collect it), this substance makes magical herbs and flowers grow extremely well (FB)
First Year
Students work only in greenhouse 1. They are lectured about such dangerous plants as Devil's Snare, but do not actually work with very dangerous plants (PS8, PS16).
Second Year
Students begin working in greenhouse 3 with more dangerous plants that require protective gear for safe handling, such as mandrakes, which when fully grown can kill the unwary (CS6). Among the practical lessons in Herbology this year students are taught how to prune Abyssinian shrivelfigs CS15).
Third Year
Students have practical lessons in learning to harvest puffapods (PA8).
Fourth Year
Students learn the properties of bubotuber pus and have practical lessons in how to collect it safely (GF13).
Fifth Year
Like most of the other teachers, Professor Sprout begins her fifth year classes by lecturing the students briefly on the importance of their O.W.L. before beginning a year of loading them down with homework, which even in this class involves essays (OP13). At the end of this year students take the O.W.L., which includes a practical portion that requires each student to work with some selection of plants. Harry, for example, was bitten by a Fanged Geranium during his exam (OP31).
Sixth Year
Students who have achieved an O.W.L. pass grade in Herbology that meets Professor Sprout's standards may continue taking the subject this year in preparation for the N.E.W.T. . Harry, Ron, Hermione and Neville all took Herbology in 6th year, working with "more dangerous" plants (HBP11).
Seventh Year
Students prepare to take the N.E.W.T. in Herbology, the highest level of qualification offered by Hogwarts.