Rich Media Ad Formats
They are called animated overlay, or floating, ads. They are sometimes called "takeover" ads and they are among a new breed of "rich media ad formats."
These ads end up on your screen because the web site owner, the content provider, has placed them there. They make money by obliterating the content you are seeking. They literally take away what you want in an attempt to get you to buy something. And they often have nothing to do with why you went to that web site in the first place.
They are the Internet version of public broadcasting pledge drives.