Stage 1 -  For child who will swim with aids such as a body belt, noodle or armbands. If you are first time swimmer, this is the place to start! 
Stage 2 -  Swimmers in this class are out of armbands and body belts. In general they can swim short distances on their front and are learning backstroke and breaststroke.
Stage 3 -  Once you have made Stage 3 you are competent at swimming on front and back. You might still be learning some of the trickier techniques such as breathing to the side on front crawl and mastering the breaststoke technique.
Stage 4 -  All refining your technique at this point and getting ready for the big pool!