Absolute line numbers

you can see simple absolute number

:set nu

if you wan't simple absolute number

: set nu!

You can do this, but you need to configure vimrc for continued configuration.

vi ~/.vimrc

set number

Relative line numbers

with the relativenumber option .
each line in your file is numbered relative to the cursor's current position to show the distance to that line.
the current line is marked 0 , the ones above and velow it are kared 1 , and so on

turn relative line numbers on

:set relativenumber
or 
:set rnu

turn relative line numbers off

:set norelativenumber
:set nornu

Hybrid line numbers

since vim 7.4 , enabling number and relativenumber at the same time produces hybrid line number mode . All lines will show their relative number ,
except for current line , which will show its absolute line number.

turn hybrid line numbers on

:set number relativenumber
:set nu rnu

turn hybrid line numbers off

:set nonumber norelativenumber
:set nonu norun

vi vimrc

if you want configure vimrc

set number 
set relativenumber