Help me make sense of small businss tax forms!

Starting a small, part-time, home-based eBay business in New York. What are the implications of registering for a Sale & Use Tax certificate? It looks like I'm going to need this certificate (DTF-17, I believe) before I deal with most wholesalers. I'm having trouble wrapping my head around the purpose/implications of all these wonky tax forms, especially the "sales & use" tax. Could somebody possibly explain this one to me in my right-brained kind of English? What will it mean once tax time rolls around in April? Unfortunately, I was not born with a gift for this kind of stuff, and I envy all of you who were. So thanks in advance for your big brains!