21 Days is a free lifestyle application developed by CodeCraftz for Android devices. The app is designed to help individuals develop good habits and get rid of bad ones. The theory behind the app is that it takes 21 days to learn a new habit, and this is where the app gets its name.

The app allows users to define their challenge by choosing a habit they want to create or get rid of. Users can then track their progress by updating their progress each day. The app also provides reminders to help users stay on track with their challenge. Additionally, users can share their progress with their Facebook friends, adding a social element to their habit-forming journey.