It requires some means whereby you send your moves to opponents and in return receive their replies. In the past this usually meant the postal service and postal chess will remain very important for many years yet. However, it is wrong to say that 'postal chess' as a synonym for 'correspondence chess'. The rapid development of e-mail chess is speeding up play and promises to make CC much more rewarding for its players. The broader term 'correspondence Chess' is harder to define, and it is also rather a mouthful.
Correspondence chess embraces any mode of move transmission.
The actual method of sending moves is in most cases irrelevant, except that transmission times have in the past had a large bearing on the nature of CC play. The number of games a player can handle depends in part on how rapidly replies are coming back. You analyse games and decide on your moves at a time of your choosing that can be fitted into the busiest work or domestic schedule. CC is played in your own home. CC offers a good route back into competitive chess for people thinking of taking up chess again after a long interval.