CTFU is an accronym which stands for "Crack(ing) the fuck up". CTFU is very similar lol and LMFAO, and it's often used to replace LMAO.
Though CTFU is mostly used to mean 'Crack the f*ck up', this accronym also has other definitions such as "Cheer The F*ck Up" among others.
CTFU Example
After reading a story about a guy talking about how many chicks he get, someone might comment "CTFU this dude looks like Steve Urkel".