skill
snice
which
whence
locate
what are the use of these command
An easy way to know is to type the following in a unix shell. That's the exhaustive solution you can get.
man command-name
If I am not mistaken, skill and snice are now obsolete and were used to send some signals about the process status.
which is used to locate the path of a command. Ex: which ls
whence is similar to which but is not found in all distributions
locate is used to locate files by their name