Coins are graded on a scale of 70, with grades of 60 through 70 being reserved for uncirculated. In theory, a coin has grade 70 if it is perfect. Many coin collectors hold firmly that there is no such thing, and restrict their purchases to grades 69 and lower. And, they get a huge price break when they do this.
In addition to the number are letters, such as PF or MS. PF means the coin has a proof finish, and MS is used for all other coins, with the letters standing for Mint State.
Special designations such as Reverse Proof, Cameo, and Deep Cameo are also important.