To kill processes in Linux several options can be used.

kill command

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The most important and frequently used option is kill command.
The kill command is used to terminate the processes on Linux and several other Unix like systems.
The kill command enables us to kill a process without logging out or restarting the computer.
Each process (also called task sometimes) on Linux  have its own unique process identification number (PID).
The PID is assigned to the process as soon as it is created.

The kill command has following syntax:

kill [options or signals] [...]

The kill command in its simplest for can be as follows:

$ kill 1565

Where 1565 in the above command is the PID of the process.

The PID of a process can be found in following ways:

Use ps command to find the PID of process
To print all the processes and their corresponding PID, run the following command:

$ ps -A

The sample output of the above command can be as follows.

PID TTY TIME CMD
1 ? 00:01:08 systemd
2 ? 00:00:00 kthreadd
3 ? 00:00:07 ksoftirqd/0
5 ? 00:00:00 kworker/0:0H
7 ? 00:04:22 rcu_sched
8 ? 00:00:00 rcu_bh
9 ? 00:00:02 migration/0
10 ? 00:00:12 watchdog/0
11 ? 00:00:11 watchdog/1
12 ? 00:00:02 migration/1
13 ? 00:00:11 ksoftirqd/1
15 ? 00:00:00 kworker/1:0H
16 ? 00:00:11 watchdog/2
17 ? 00:00:02 migration/2
18 ? 00:00:05 ksoftirqd/2
20 ? 00:00:00 kworker/2:0H
21 ? 00:00:10 watchdog/3
22 ? 00:00:02 migration/3
23 ? 00:00:17 ksoftirqd/3
25 ? 00:00:00 kworker/3:0H
26 ? 00:00:00 kdevtmpfs
27 ? 00:00:00 netns
28 ? 00:00:00 perf
29 ? 00:00:03 khungtaskd
30 ? 00:00:00 writeback
31 ? 00:00:00 ksmd
32 ? 00:03:40 khugepaged
33 ? 00:00:00 crypto
34 ? 00:00:00 kintegrityd
35 ? 00:00:00 bioset
36 ? 00:00:00 kblockd
37 ? 00:00:00 ata_sff
38 ? 00:00:00 md

If you know the process name you can get its PID using the grep on ps command.
For example you want to know the PID of process whose name is psql the following command will give you the PID of the psql.

$ ps aux | grep psql

The sample output of the above command can be as follows.

user 6763 0.0 0.0 113292 6756 pts/31 S+ Feb28 0:00 /usr/lib/postgresql/9.5/bin/psql -U user_name platformdb_test
user 15078 0.0 0.0 15756 1036 pts/28 R+ 12:34 0:00 grep --color=auto psql
user 18735 0.0 0.1 113292 8612 pts/27 S+ Mär10 0:00 /usr/lib/postgresql/9.5/bin/psql -U user_name --host=127.0.1.1 --port=34453 platformdb

Use pidof command to find the PID of process
For example you want to know the PID of process whose name is psql the following command will give you the PID of the psql.

$ pidof psql

Output

1658

The following command can be use to kill process based upon PID

kill -9 1656

or

kill -SIGTERM 1656

where pid=1656.
-9 flag in the above command is SIGKILL.
The default signal for kill is SIGTERM.

To kill more then one processes use the following command

# kill PID1 PID2 PID3

or

# kill -9 PID1 PID2 PID3

or

# kill -SIGKILL PID1 PID2 PID3

where PID1, PID2, PID3 are the PID of the processes.

To list all the signal names with their corresponding number use following command

$ kill -l

Output:

1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP
6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1
11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
16) SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ
26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR
31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3
38) SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8
43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7
58) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2

some other examples
kill -9 -1
Kill all processes you can kill.

kill -l 11
Translate number 11 into a signal name.

kill -L
List the available signal choices in a nice table.

kill 123 543 2341 3453
Send the default signal, SIGTERM, to all those processes.

kill -legal
List signal names. This option has optional argument, which will convert signal number to signal
name, or other way round.

killall command

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killall cammand can be used to kill processes by their name.

killall syntex:

killall [-Z, --context pattern] [-e, --exact] [-g, --process-group] [-i, --interactive] [-o, --older-than TIME] [-q, --quiet]
[-r, --regexp] [-s, --signal SIGNAL, -SIGNAL] [-u, --user user] [-v, --verbose] [-w, --wait] [-y, --younger-than TIME]
[-I, --ignore-case] [-V, --version] [--] name ...

killall sends a signal to all processes running any of the specified commands. If no signal name is specified, SIGTERM is sent.
Signals can be specified either by name (e.g. -HUP or -SIGHUP) or by number (e.g. -1) or by option -s.

Example:
For example you want to kill the process whose name is psql. use the following command:

killall psql

pkill command

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pkill command look up or signal processes based on name and other attributes.

syntex
pkill [options] pattern

Example:
Send SIGTERM to all the process which has psql in its name.

$ pkill psql

before giving the above command we can find which processes are fulfilling the above criteria by following command:

$ pgrep -l psql
6763 psql
18735 psql

$ pkill psql

We can find the status of a process by using one of the command below:

$ service psql status
$ pgrep psql
$ ps -aux | grep psql