Basically, they are fake profiles set up on Craigslist and virtually every dating or social networking website that display pictures of too-hot-to-be-true women, often in very revealing clothing and/or suggestive poses. If you try to contact one of these "people", you receive a fake message in which you are linked to a seperate website where you can supposedly watch these "girls" perform naughtily for you on their webcams. The kicker is they will make a big deal of telling you it's all 100% free....only they need your credit card information for "identity confirmation" purposes or some bullshit.
If you browse websites like that long enough, it becomes very easy to spot these pains in the ass.