To add what Derek said. Single Quinta (SQ) ports are from the top vineyards (or Quinta) for a given producer. Taylor Fladgate has Quinta de Vargellas, Dow has Quinta do Bofim, etc. When a house (i.e. Taylor, Fonseca, etc) doesn't "declare" a Vintage Port (VP) in a given year, but when the quality is high enough (but not high enough to declare) they often bottle SQVP's. These ports are not a blend of several vinyards (Quinta's) but bottled from grapes of a single vineyard, hence the Quinta name on the bottle.
Usually, they are great values to buy between declared years. BTW, a great book to read, and keep as a reference, is "Port and the Douro" by Richard Mason. I refer to it regularly.
EDIT: Looks like Tyler beat me to it, good explaination
