Memes are funny things that get reposed everywhere on the Internets.
Class 1 memes (True memes) are photos with impact font text on the top and bottom. There are occasionally ones with multiple panels, but that is rare. Class 1 memes fall into 3 g: The One hits, where the photo they put the text on is only used in that one meme; The Small times, which have a name and the photo appears in multiple memes despite not being famous (such as Laundry Room Viking or Religion Pigeon) and finally the Kings, which everyone knows (such as Good Guy Greg, Philosiraptor, or Success Kid). For a list of class 1 memes, go here.
Class 2 memes (troll memes) are black and white comic strips where the characters have poorly-drawn, often stick figure bodies, but very detailed heads. They very loosely fall into three groups that often intermingle: Troll comics, which are about you "trolling" someone (pranking, baiting, etc) Rage comics, which are about someone trolling you, and Other comics, which aren't about either.
Class 3 memes (fake memes) aren't actually memes, but are instead things that are popular on the internet. for example: doge, nyan cat, troll face, MLG, shrek. They include 3 categories: Funny, Scary (creepypasta), and Vine.