They're usually time share and/or travel agency offers... generally you get the trip for "free" but you have to pay a fortune in "fees and taxes" to get the "free" trip - sometimes they'll do things like overcharge then ask for your bank account info so they can refund directly to you... there's all types of them...
Here's one article on the scams:
http://www.infoplease.com/ipa/A0004591.html