This is why it's important to be concrete and specific, because (as the debate between thir and Rose is demonstrating) people understand very different things by words like "sadist." Instead of saying "I'm a masochist," say "I get off on being hurt" (or, if you're still learning, "I get off on fantasies of being hurt.") Describe fantasies, tell what scenes you already know work for you, say what you've tried and what you want to try. Leave the psychological classifications for later when you're talking just for fun.
If it's hard to speak freely about your tastes, which is common enough, get hold of one of the BDSM Checklists that float around the Web, and you and the prospective playmate/partner fill them in together. The fact that someone (say) checks things involving dominance and control rather than those involving pain and bondage will tell you far more than trying to work out the distinctions between a sub, a bottom and a masochist.