Irony is a type of comedy humor, where something that is supposed to be one way is the complete opposite.
Example from Get Fuzzy:
Bucky: If you drop that cake, Satchel, it will be on your head!
(Satchel drops the cake on Bucky's head)
Satchel: ...oh. How ironic.
Irony is viewed as one of the highest types of humor when correctly pulled off. However, if not used correctly, irony can also be just as un-hilarious.