cd [directory]
Will change directory to the [directory]

top
This shows basic information such as uptime, cpu usage, memory usage and processes running.

ls
List all files in a directory.

ls -al
List all files plus hidden files.

ls -alh
List all files and hidden files and display the file size in an easier format.

ps -aux
Show all processes running.

tail [filename]
Show the end contents of a file. This is good for viewing last entries to log files.

who
Show who is logged into shell.

last
Show a list of all accesses to the server ftp and shell.

pwd
shows your current position in the filesystem

locate [filename]
Will find all files named with [filename]

chmod 755 [file]
Will change the permissions on [file]

cp [file1] [file2]
Copies [file1] to [file2]

rm [file]
Deletes [file]

exit
Logs you out of shell.