To kill all the processes that you have the permission to kill, simply run the command

kill -15 -1 or kill -9 -1 depending on the desired behavior (use man kill for details)

To kill a specific process, say, firefox, simply run

pkill firefox or killall firefox depending on the behavior you want: What's the difference between 'killall' and 'pkill'?

If you want to see what processes are running use the command

ps -ef

If you want to look up all processes by user bob, this might help

pgrep -l -u bob

or

ps -ef | grep bob

There are many ways to go about this.

Method #1 - ps

You can use the ps command to find the process ID for this process and then use the PID to kill the process.

Example

$ ps -eaf | grep [w]get 
saml      1713  1709  0 Dec10 pts/0    00:00:00 wget ...

$ kill 1713

Method #2 - pgrep

You can also find the process ID using pgrep.

Example

$ pgrep wget
1234

$ kill 1234

Method #3 - pkill

If you're sure it's the only wget you've run you can use the command pkill to kill the job by name.

Example

$ pkill wget

Method #4 - jobs

If you're in the same shell from where you ran the job that's now backgrounded. You can check if it's running still using the jobs command, and also kill it by its job number.

Example

My fake job, sleep.

$ sleep 100 &
[1] 4542

Find it's job number. NOTE: the number 4542 is the process ID.

$ jobs
[1]+  Running                 sleep 100 &

$ kill %1
[1]+  Terminated              sleep 100

Method #5 - fg

You can bring a backgrounded job back to the foreground using the fg command.

Example

Fake job, sleep.

$ sleep 100 &
[1] 4650

Get the job's number.

$ jobs
[1]+  Running                 sleep 100 &

Bring job #1 back to the foreground, and then use Ctrl+C.

$ fg 1
sleep 100
^C
$

pkill firefox

More information: http://linux.about.com/library/cmd/blcmdl1_pkill.htm