Where sign up?

Lets use Facebook for an example:

Sign In: If you sign in to Facebook you already have an account. You are just typing in your log-in information.

Sign Up: If you are signing up for Facebook it means that you don't have an account yet and are making one. You create your log-in information. After you have signed up you will always log-in to Facebook after that. Unless of course you are creating a second account.

SIGN IN:Already have an account and are putting in your information

SIGN UP:You are MAKING an account and are still giving your information

So there you have it!! Sign in and Sign up:

You can do the same for this website aswell! ;)