What we call "english" riding is the way that most of Europe rides, and other areas of the world, too. it means basically that you ride with a lighter seat and utilize the stirrup to stand up off the saddle at times to lighten the horse's back for movements like jumping or going up/down terrain. Do not think that E riding is "wussy" . it is so not! it takes a lot more muscles and if you ever see folks riding out to the foxhunt, you will see some crazy hard riding. There is more of an emphasis on sitting up and out of the horse's way, with more drive forward in the hrose's movement, less collection , in a general sense.
Dressage might , from the point of view of one not familiar with it, be more likened to western riding , such as reining. The rider sits more down into the horse, like W riders do. And there is more emphasis on having the horse stay balanced and under the rider, rather than the rider staying out of the horse's way as it runs across country. Mind you, these are real generalizations. All these sports are much more complex and refined than I am making it sound. But you'll find you own way there and I will be interested in hearing your impressions of the differeneces.