What is the difference between “als” and “wie”?

The difference is basically the same as between “than” and “as”, respectively. You would use “als” when comparing two things that aren’t on the same level ( for example: better THAN = besser ALS). “Wie” on the other hand would be used if the two compared are on the same level (for example: Just as good AS = Genauso gut WIE)