When you see 'BU' or Brilliant Uncirculated, you're usually looking at the lowest (MS60) grade that can still be called uncirculated. Normally (with dollars, anyway) a BU will have a whole mess of bag marks. As long as there is no wear, it is considered uncirculated or 'BU'. I think BU should stand for 'Barely Uncirculated'.

Like Larry said, a 'slider' is a very good 'AU' in which the seller is trying to get 'BU' money.