Played with rules identical to chess or chess variants, but with an element of spatial and temporal separation!
Traditionally, players will send each other moves by mail.
After receiving the opponent's move, one has a considerable amount of time to respond compared to over-the-board chess. An entire game may take months or years to complete!
(Pictured here is a simple postcard used to play correspondence chess.)
The chess community is often divided on the issue.
Some are in favor of enforcing "no computers allowed" play, often with the belief that use of chess engines devolves the game into a battle between computers, not humans.
Others prefer the use of chess engines to augment and enhance their play! (Human-Machine merger)
Garry Kasparov, a world chess champion has said that a human+machine 'centaur' could outplay both a computer alone, and a human alone.
Garry Kasparov; remember this guy? He was in the class lectures regarding A.I.