The main difference is the saddle. English saddle is generally a lot lighter and has no horn. Western saddle is heavier and does have a horn. You can also walk, trot, canter, and gallop on both. Another main difference is the way you sit on it. English you have to use pressure everywhere, so it's not as comfortable for the horse. Western saddles are much more deep and much more comfortable. And Western riding is a LOT faster...TRUST ME. :D
Hope this helps! :)