Cheating is the act of doing things not intended by developers or considered to be against the "spirit" of the game.
- Using wizard mode to test things out, before trying it in a legit game.
- Reading spoilers
- All forms of scumming
- Deleting level files
Save scumming is creating a back-up save file in case your character dies. Thus, eliminating the permanency of death.
Selectively deleting ghost files
It is possible to delete ghost files. If you do this selectively, deleting only the ones that you think will kill you again, this is cheating. If you always delete all ghost files including ones you would be glad to find, this is not cheating as it is the equivalent of playing every game on a fresh install.
Leaving weak ghosts behind
When players die they may leave a ghost on that level. Players can create high level characters and give them major weaknesses, before killing them. When they encounter the weak ghost later on, they can kill them and get tons of XP early on.
Intentionally crashing the game
Crawl only autosaves every level, so when a crash occurs, you will be dropped at the last time you entered the current level. On a local system, this is easily done by force-quitting the program. However, it is also possible even on public servers, if you know of a bug that allows you to produce a crash on demand. The devteam fixes these bugs whenever they find them, and furthermore keeps reports of such bugs private until they are resolved. The secrecy is necessary because intentional crashing could conceivably be used to gain an unfair advantage in a tournament.
Obviously, restoring a game after an accidental crash, even on a public server, is not cheating. Also note that losing your connection to a server in the middle of a game will not cause a crash - you will restore exactly where you left off.