This function extracts and tabulates the entities used across the entire dataset. In Twitter parlance, an entity is an occurrence of a screen name (called a mention), hashtag, or URL within the text of a tweet. Multiple entities can be mixed and matched within any given Tweet. By looking at the top users, you can get a sense of influential users; by looking at hashtags, you can start to draw out a qualitative sense of the streams contents; by looking at the URLs, you can start to draw out the connection the group of users have to the rest of the internet.