sed reads the standard input into the pattern space, 
performs a sequence of editing commands on the pattern 
space, then writes the pattern space to STDOUT. 

sed is a command used for opening stream editor.

sed is a stream editor for editing files from a script or from 
the command line

its same like Find and replace strings

sed -e 's/oldstuff/newstuff/g' inputFileName > 
outputFileName

sed is a multipurpose tool which combines the work of 
several filters.

sed uses instructions to act on text.
AN instruction combines an address for selecting lines with 
an action to be taken on then , as shown by the syntax
  
 sed options 'address action' file(s)

sed reads the standard input into the pattern space, 
performs a sequence of editing commands on the pattern 
space, then writes the pattern space to STDOUT.