Definitions

Stamp collectors will be familiar with these definitions but other visitors may not be. I'm not going to look up the 'book' definitions .. just going to give my quick thoughts

Definitive Stamp: A stamp that is part of a series and is used for many years. Often referred to as the 'workhorse' stamps. Usually smaller in size and often reprinted .. thus leading to many varieties of the same stamp. Most US Flag stamps are definitive stamps.

three different varieties of the Wavy Flag series issued 2002-05

Commemorative Stamp: A stamp that was issued to commemorate some event, person, or institution. Usually larger than a definitive stamp and rarely reprinted (so far fewer varieties). There are a few US Flag stamps that are commemorative.

Scott #949 Issued in 1945

Cover: A piece of mail with stamp affixed. Usually a letter or a postcard. Depending on time and/or place, may be highly collectible.

not a flag stamp but illustrates the definition ... interesting (and perhaps collectible) due to it's age (postmarked 1888)

First Day Cover (FDC): A cover issued by the post office as a collectible. Usually issued on the first day that a stamp becomes available to the public and will almost always have some sort of artwork (called a cachet) on the cover. Not usually as valuable although a few can become collectible depending on circumstance. Look up First Flight covers, Naval Covers, etc. for examples.

FDC Air Post Postal Card Scott # UXC8 issued in 1966