Pusher=someone who sells for money first
Dealer=someone who sells to make sure people have a good time first
to elaborate, a pusher will always let you know when he (or she, but seems to usually be hes) has drugs, what kind, how much, whether you want them or not. While this means that you know where to get stuff, this can get annoying. hence "push" in pusher
a dealer, OTOH, is someone who deals in a limited selection of high quality products, and generally has a much smaller client base. You have to call them, and they may not always have what you want in stock, but when they do, it will generally be of higher quality, although the price might be higher as well. hence, you have to "deal" with them