What is sexism?

I mean really. Every time Palin is challenged, the McCain campaign screams sexism. I really have no clue wtf sexism is obviously. Since when is it sexism to call the kettle, "black"?

I fail to see how vetting a person that could a heartbeat away from the presidency, constitutes sexism. This is a constructive question and I'll certainly listen to examples.

Kindly just explain what constitutes sexism in the context of Sarah Palin.

Thanks!