Any action that you perform regularly as part of your routine can be called a habit. Flossing your teeth would be considered a good habit. Sucking your thumb would be considered a bad habit, especially as an adult.
On New Year's people always vow to break their bad habits, but bad habits die hard. The noun habit can also refer to an established custom — like the habit of breaking bread with your family every holiday. On a completely different note, clothing that is worn for horseback riding, or by a nun or monk, can also be called a habit. All good nuns should be in the habit of washing their habits.