You remember the TV show What's happening? Well now you can answer the question on Twitter as they have switched from "What are you doing" to What's happening?
From the Twitter Blog: What's Happening
Twitter was originally conceived as a mobile status update service—an easy way to keep in touch with people in your life by sending and receiving short, frequent answers to one question, "What are you doing?" However, when we implemented the service, we chose to leave something out. To stay simple, Twitter did not require individuals to confirm relationships. Instead, we left things open.