There is one question you need to ask first: Why do you have a Twitter account?
Depending on this answer, your “what” will change tremendously. If you have a twitter account to express yourself — you will try to capture your essence in it. If you have it in order to sell your goods or services — your tagline will appeal to your potential customers. If you want to connect with people doing/loving same thing as you — it will be reflected too.
Stephanie, the problem you are talking about comes to existence only if you have no answer to “why” question. When this happens, we tend to put into our profile an answer for irrelevant stranger to one of the implied questions: “Who are you?” or “What do you do?”. You might try to appear smart/witty/funny or go with safe/everyday answer.
So, why do you have your twitter account?