1. Used to filter and modify text (since SED is Stream EDitor)
2. It is just like searching for a text and replacing it within vim. To replace the text with 'sed' we simply use:

   sed 's/old_text/new_text/' filename
   
   But filename itself will not get modified, it will just show the file with replacements. To make changes to file, we use
   sed -i 's/old_text/new_text/' filename
   

3. The above does work with different delimeters ('/','.',' ','|')

   i.e. we will get correct answer with 
   sed -i 's old_text new_text ' filename
   sed -i 's|old_text|new_text|' filename
   sed -i 's.old_text.new_text.' filename
   We can use these delimeters when old_text which is to be replaced itself contains other delimeter ('/').
   in that case if we want to replace '/etc' with nothing we will use:  sed -i './etc..' filename
   in that case if we want to replace '/etc' with nothing we will use:  sed -i '|/etc||' filename
   in that case if we want to replace '/etc' with nothing we will use:  sed -i './etc..' filename
   
   

A special character which followes letter 's' becomes the delimeter.

example: echo hi there | sed 's/hi/hello/' will print : hello there