This English grammar lesson explains the concept of would have could have should have and its different forms (would've could've should've-often mistakenly misspelled would of could of should of because that is how it sounds- and woulda coulda shoulda)
When you are finished with this grammar tutorial, continue to the practice exercises to practice the different forms and the meaning. There are more illustrations and information at
http://www.improveenglishesl.com/intermediate-wouldhave-couldhave-shouldhave.html