Prior to 1967 covers generally fell into 2 categories: plain or illustrated. Subsequent to this date there were ordinary or official covers.
Ordinary (or commercial) covers were produced to allow collectors to purchase them before the day of issue and affix the stamps themselves. They would then have to arrange to get them franked with a first day of issue postmark.
Official covers, also known as sponsored covers, are covers specially produced by organisations or cover producers and who at the same time also sponsor a special handstamp for use on the cover.
Often these covers did not have the full set of stamps being issued on that day as only one or two values may be relevant to that organisation. Consequently, privately sponsored covers, bearing the full set of stamps are often more valuable than ordinary covers with the same handstamp due to their rarity.
Illustrated and Plain Covers
These terms are mainly applicable to covers between 1924 - 1961. Most covers these days are illustrated i.e. the envelop bears some design, picture or motif on it. Because of their rarity, First Day of Issues of earlier stamps can be just as valuable as illustrated covers and indeed, some stamps are only found on plain covers.