Harry Potter fans please come hear and help me!?

ok i found this picture of James,Sirius, and Lupin when they were in Hogwarts. it shows James playing with a snitch. no big deal right? ok well i sent it to my friend and she luved it. but she says actually he is a chaser not a seeker. so i said ok whatver. the problem is i can't remember what the chaser's role is in Quiddetch(did i spell it right?) what do they do?