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.
-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