Cosplaying is pretending to be someone who exists in an existing anime or manga. (example: pretending to be Light Yagami in Death Note, or your favorite Pokemon.)

Roleplaying is being someone you created in either an existing anime or manga, or someone in something you created. (example: I like to play that I am a level 9 captain from Bleach who likes to walk around in Bankai, or come up with your own story for others to participate in.)