The other commenters covered it but before Politically Incorrect and Real Time he was a minor comedian in the 80s. He did standup, bit parts on a few TV shows, and I think he got a movie.

So he had a career to speak of before he became primarily a political commentator. His standup act was very reactionary and critical of the "growing trend" (at the time) of political correctness/basic human decency, which is how he got his first show.

Edit: Oh, I forgot to mention he is somewhat left-leaning. He claims to be a liberal, or rather people claim he is, but his stances are all over the place. He's mostly cultivated a brand of a smug asshole who would call out hypocrisy while being hypocritical..but it's "okay" because he's open about being a hypocrite?

He tried to fill the role of THE democrat-hard-hitting-common-sense-political-comedian but was quickly overtaken by Jon Stewart on The Daily Show. In my opinion Bill Maher never truly fulfilled that role because he lacks empathy.