Cal's greatest strength is organizing your daily agenda, basically all of the to-dos, meetings, and appointments you have on your plate on one given date. When you launch the app, it automatically jumps to today's agenda, which shows all of your events and Any.do tasks in a chronological list.
Along with the standard notification in the top status bar, the Android app comes with special notifications that pop up at the bottom of your phone's home screen. They show how much time until your next item starts, who's attending, plus links for navigation and Uber, if applicable. I like that these notifications give me enough information about the event without needing to open the app.