Use the command ps to display currently running processes.

ps -aux will show all processes under all users. In order to kill a certain process, just type:

kill -s KILL [process ID]

The process ID will be listed in the pid column when you run ps -aux.  For example:

root@demo [/etc]# ps aux
USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0  1516  160 ?        S    Aug29   2:07 init [3]         
root         2  0.0  0.0     0    0 ?        SW   Aug29   0:00 [migration/0]
root         3  0.0  0.0     0    0 ?        SW   Aug29   0:00 [keventd]
root         4  0.0  0.0     0    0 ?        SW   Aug29  10:13 [kapmd]
root         5  0.0  0.0     0    0 ?        SWN  Aug29   0:00 [ksoftirqd/0]
root         8  0.0  0.0     0    0 ?        SW   Aug29   0:00 [bdflush]
root         6  0.0  0.0     0    0 ?        SW   Aug29  14:15 [kswapd]
root         7  0.0  0.0     0    0 ?        SW   Aug29   5:17 [kscand]
root         9  0.0  0.0     0    0 ?        SW   Aug29   1:44 [kupdated]
daemon    2790  0.0  0.0  1576  244 ?        S    Aug29   0:03 /usr/sbin/atd
...
root@demo [/etc]#

You can also kill all processes under a certain name, using as

killall [process name]