cd

Change directory

ls

Show contents of directory

mkdir

Create a directory

rmdir

Delete a directory

rm                      

Delete file (or directory)

su

Set User. Allows changing to specific user. With no parameters, changes to root

top

Gives information about the system load

ps

Show running processes

kill

End a process(es)

df

List the amount of free space on disks

netstat

Information about the current networking status

gzip

Compress/Decompress files

tar

Archiver (combine many files to a single one)

bz2

Compress/Decompress. A bit better then gzip (and newer)

man

Manual page. Supplies information about command

du

Disk Usage. Returns information about "how much disk space" is used by a directory

echo

Output some value to the std output.

which

Show the path of specific program