[Hugh]

Hi ... just to add to what Roy said, the term 'cover' harken's back to the days when postage rates were paid based on weight. To save on postage, people just folded their letters in such a way to created a cover on which they could write the address where it was being sent  The first use of envelopes dates to the early days (1840+) of adhesive stamps when weight wasn't quite so important. However, the term cover is still used for any kind of item that goes through the mail, as Roy said. 

'Cover' collecting isn't limited to First Day covers .. people also collect First Flight Covers (the first day when mail is sent by air along a new route), Event Covers (e.g. a CNE cover or a cover issued to celebrate a stamp exhibition), a Commercial Cover (which Roy collects, he has an amazing collection), Advertising Covers, Dear Doctor Covers (usually as postcards), Stamp-less Covers (early covers before the use of adhesive stamps), Postal Stationary Covers (with printed indicia instead of adhesive stamps), Railroad Covers, Postage Meter Covers, Censored Covers, POW Covers, Patriotic Covers ... the list is endless.

It's a fun area to collect. 

Regards, Hugh