First, there are two registers: the Principal Register and the Supplemental Register. Registration of a mark on the Principal Register conveys the important substantive rights that most people associate with federal registration and, as a result, it is the preferred method of federal trademark protection.
Probably the most important benefit of placing a mark on the Principal Register is that anybody who later initiates use of the same or a confusingly similar trademark may be presumed by the courts to be a "willful infringer" and therefore liable for large money damages.
Registration on the Supplemental Register does not convey the bundle of rights and protections granted on the Principal Register. For example, registration on the Supplemental Register is not evidence of the owner's exclusive right to use the mark in connection with the goods or services, and the owner of a mark on the Supplemental Register cannot utilize the power of the Customs services to stop importation of infringing goods.