Correspondence Chess vs. Over-the-Board
Correspondence chess and over-the-board (OTB) chess are almost different games. One gives you days to think about each move. The other gives you minutes. Understanding the differences helps you appreciate both formats.
Time per move
Correspondence: days or weeks per move. Games can last months or years. OTB: minutes per move in classical, seconds in blitz. A single game is completed in one sitting.
Engine use
The biggest difference: in modern correspondence chess, engine use is allowed (and universal). Games are essentially human-guided engine battles. In OTB chess, any form of computer assistance is cheating. This means correspondence chess tests preparation, evaluation, and understanding of engine suggestions, while OTB tests pure human calculation.
Which is "harder"?
They test different skills. OTB is harder on calculation and time management. Correspondence is harder on preparation and deep positional evaluation. The best OTB players are not necessarily the best correspondence players, and vice versa.