PostmarkA postmark is a cancellation or marking made on a postage stamp, letter or package indicating its approximate date of use. (The postmark must be distinguished from the earliest cancellations, which simply cross out the stamps with a pen.) Postmarks can be either applied by hand or machine.
A postmark should not be confused with the killer which are lines, bars, etc. used to cancel a postage stamp. Neither should a postmark be confused with pre-cancels (stamps that have been cancelled before the envelope or package to which they are affixed is submitted or deposited for acceptance into the mailstream, they most commonly have taken the form of a pre-printed city name on the stamp), which generally do not indicate a date.
Flight cachets, more or less elaborate rubber-stamps on an envelope indicating on which flight (typically a first flight) a cover has traveled via air mail, are in addition to the postmark and are not postmarks either.
Datestamp is a type of postmark.
Postmarks were introduced to help track the delivery quality/speed of the postal service. In the 1800s and early 1900s it was common for letters to receive multiple postmarks indicating the time, date and location of each post office delivering/transporting the letter. (While postmarks are applied almost universally by or under the authority of the official postal department, service or authority, it is at least theoretically possible that under certain conditions specified by the private express statutes in the United States, a privately-carried letter may be cancelled with a private postmark.)
A special or rare postmark can substantially add to the value of a stamp. (In addition to everyday postmarks there are postmarks indicating the first day of issue of a particular stamp and pictorial cancellations commemorating local events, anniversaries and the like.)
Much of the published work on postmarks covers postmarks from before 1900. (This is perhaps because in the United States so-called fancy cancels were prevalent in this period, with the cancelling device often hand-cut from cork by the postmaster in elaborate shapes such as flags, stars or shapes that were seasonally-appropriate such as turkeys for Thanksgiving). Much work in studying postmarks is needed for 1900 and later.
Fewer postmarks are used now than previously, with the advent of meter labels, which indicate the precise date and time of acceptance at the post office, some types of computer vended postage, and computerized postage people can print off their own PCss (called in the United States PC Postage, these services were offered by such companies as Stamps.com and Neopost, Inc.).
The Postmark Award is given to outstanding employees of Canada Post.