Meme (Pronounced "Meem") is an internet phenomenon created at (or adopted by (se video)) the imageboard /b/ at 4chan. Memes often describes funny, informing, pranking, mathematic (etc.) occurrences on the internet.
To get a clearer view of the meme phenomenon, look at this youtube clip.
“Some memes you might recognize”, they all come from (4)chan (sites).
“But mom! Where does the meme come from?”
Memes are often born accidental. If a single person tries to force a meme into the system of memes, it’ll probably be called a “forced meme”. Its unlikely that it'll retain its title as a meme. The most popular memes are created when the result of a thread is (very) funny, and the memory of it can be compressed in a picture with just a line (some lines) of text.The first meme created was the lolcat meme. It consisted of a funny picture of a cat, with a text added at the bottom. A famous lolcat picture is the “I Can Has Cheezburger?”.
Saturdays were declared as “caturdays”, which meant that the Anonymous should post pictures of cats (often memes). But then there came a change, "From this day forth, every day shall be Caturday" the Anonymous declared, and so it became. Nowadays caturdays mostly occurs at Saturdays, to maintain the old traditions.