It's that feelin' you get when you got high-res, glossy printouts of tub girl and goatse and you're all alone on a school bus.
Webcest is when you find dirty pictures of your ex-girlfriend on the internet and you print them out for later.
Webcest is when you realize that you're wanking off to a distant relative without knowing before.
Webcest is finding a porn site about a girl who looks like your exgirlfriend and bookmarking it.
Webcest is when you realize the fifteen-year-old you've been having cyber sex with is actually your niece.
You don't get STDs from Webcest, but you should.