What are they? Basically MFA sites are sites which litter their whole website with lotsa Adsense ads & other ads with very little content or sometimes none at all and then create Adwords campaign such that they can appear on the 1st few pages of Google's search results pages (SERPs). An example would be something like this:
So how do they manage to get enough profits while advertising at the same time? Well, that's because the Adsense system factors in high clickthrough rates and the cost of an ad when deciding to appear on a publisher's website. So if an MFA site has a higher CTR, it'll appear before other sites.
Currently, Google has not clamped down on this yet but I would recommend not to do this, as they have been alerted of this problem and are very aware of it and most probably trying to do something to stop this and it's not very wise to get your Adsense account shut down just for a few months of profits! But just for the sake of knowledge, these guys have proven how easy it is to make money out of it: