In this scenario I wish to format a cell if a different cell contains the text "Extra"

Firstly, select the cell to be formatted

Then, from the Home menu select the Conditional Formatting menu and then select New Rule

Select "Use a formula to determine which cells to format"

I want the cell to go green if the word "extra" is found so my formula will be 

=ISNUMBER(SEARCH("extra",A2))

please note - this method is not case sensitive so all instances of the word extra will be picked up even if it is capitalised. Also a positive result will be returned if the word is found within another word (e.g extraordinary would produce a positive result")

The formatting can then be dragged down to fill as many cells as required

Result