There are 2 types of auctions, one where the houses are foreclosed on by the lien holders, and one where already foreclosed bank owned properties are bundled together and given to an auction house to sell to the public.
At the courthouse auctions, properties that have been served a notice of default by the lender or lenders are available for auction. In order to purchase one of these properies you must have all cash and pay off the lien holders. In general, those properties are ones where the lones are greater than the values right now so the bank gets them back because no one else wants them at that price. Those houses are then either given to asset managers who hire real estate agents to sell them, or are sold at lare public auctions. If given t a realtor the homes appear on MLS.
The large auctions are done by companies like redco or ushomeauctions. The homes are usually available for showing a few week-ends before the auction and then are bid on at the auction. There are lenders available a these auctions to get you financed, and they pre-qualify potential buyers before letting you bid. they charge the buyer a fee, often 1% which is how the auction house makes their money.
If you want a little more info you can go to my wesite http://www.marcymoyer.com and click on the link Distressed Property Sales to read an article I wrote about these kinds of sales.