Most cancelled-to-order come from the old Iron Curtain countries (Poland, Czechoslovakia, Hungary, East Germany, USSR, PRC, etc.) or Arab sheikhdoms (Ajman, Manama, etc.) and will have gum on the back of them despite the fact that they seem to have been used. The cancels are usually in a corner of the stamp, so as not to interfere with the design, and they are usually circular cancels, perfectly placed.
Here is an example of some East German CTO's -- note how in most cases, the cancel is applied in an attractive way so as not to cover too much of the design area. All of these stamps will have original gum on the back of them.



Polish CTO's are usually not as attractive, on the other hand:

Do not make the mistake of confusing CTO's with "Favor Cancels", which are usually something done by an individual who wants cancelled examples for his collection, as was done in this example:

Precancels in the US are usually of the "City and State" variety. They are easily found on many different definitive stamps, and occasionally on commemoratives:

Other countries also use precancels.
Here are some from Canada:


Belgium:

France:

These stamps are cancelled before they are used, and they are usually used by large volume mailers, who have to have a permit in order to get them. The idea is they send out a big shipment of, say, 1000 pamphlets in envelopes. If they used regular stamps on them, this would be a major pain in the butt to the Post Office staff, who would likely have to hand cancel the stamps on each one. With the stamps being precancelled, the mailer could then make bundles of these, usually all for the same city in each, which would then lead to faster handling. Because the mailer was doing some of the work, he would often get a price cut on the cost of mailing.