An agnostic is a person who answers the question "does god exist?" with "I don't know".

There are two basic reasons for saying "I don't know":

1) because it's impossible to know (strong agnostic)
2) because I don't know right now but might in the future (weak agnostic)

An atheist is a person who answers the question "do you believe in god?" with the answer "no".

There are two basic reasons for saying "no":

1) because god does not exist (strong atheism)
2) because I don't actively believe in any god (weak atheism)

One can an agnostic and an atheist at the same time.