Most are familiar with a U.S. utility patent, have a seven digit number followed by a “B”:

Beginning in March 2001, applications have been published (unless the applicant requests non-publication at the time of filing.  Publications are readily distinguished by the eleven digit number — four digits representing the year, and the seven digits representing a running count for the year:

Design patents have a six digit number prefaced with a “D” and followed by an “S”:

Plant patents have a five digit number prefaced with a “PP” and followed by a “P”:
Reissue patents have a five digit number prefaced with an “RE” and followed by an “E”:

In addition to patents, there are other patent related documents that patent professionals occasionally stumble upon.  Defensive Publications were offered between April 1968 and May 8, 1985, and were replaced by Statutory Invention Registrations.  They have up to a  six digit number prefaced with a “T”:

Statutory Invention Registration is a registration of an invention pursuant to 35 USC §157, and replaced Defensive Publications.  SIRs were eliminated by the AIA in 2011. An SIR has up to a four digit number, prefaced by an “H” and followed by an “H”: