"Sure" means "yes".

A: Hey, can you help me with this?

B: Sure.

 But "sure" is more casual than "yes".

If you just answer "yes" to a question, it can sound as if you are angry:

A: Hey, can you help me with this?

B: Yes.

A: Uh... Are you annoyed or something?

"Sure" is similar to "of course". Both suggest that it's obvious and expected for the answer to be "yes".

A: Will you come to my graduation?

B: Of course!

"Of course" is a stronger response and more positive response than "sure".