AU stands for "Almost Uncirculated." It represents numeric grades 50-59, or just below uncirculated. BU stands for "Brilliant Uncirculated." In general this description is discouraged in light of numeric grading, as BU is really in the eyes of the beholder. Some feel that BU should be reserved for MS-65 or better, while others will include all the mint-state grades (60-69) under the term. AU (About Uncirculated) is the grade between XF (Extra Fine) and UNC (Uncirculated). AU graded coins will have just the lightest touch of wear on only the high points of the coin's design. BU (Brilliant Uncirculated) coins, for the most part, are called that to avoid assigning a specific numeric grade -- to let the buyer determine their own grade. In general, you will find these coins to grade in the MS-60 to MS-63 range.