Rm

short for remove. Seriously, be careful when using this. There is no undelete once you've unlinked a file (unless you've aliased rm to a mv script that moves files to a trash folder or something -- not recommended because you will eventually find yourself on a system that doesn't have that alias and make a mistake.

Common flags

-r recursive -- same as -R
-f force -- do it and damn the consequences
-d directories
-i confirm before delete
-P overwrite files (with 0xFF's, 0x00's, and 0xFF's again) before deleting them
-v be verbose

See also rmdir