Habits: can be either good or bad, just like real habits. Good Habits, denoted by a plus sign "+", are rewarded by experience and gold. Bad Habits, denoted by a minus sign "-", are penalized by health loss. A single Habit can be just good, just bad, or both. Habits that are both positive and negative can be used for goals that players can both make progress on (rewarding themselves) or fail to make progress on (penalize themselves). (Read more on Wikia)
Dailies: are used to track habits scheduled in a known, repeatable fashion. This allows a player to incorporate a habit into their everyday routine (example: working out, flossing) or into a weekly routine (example: Saturday morning call to Mom). (Read more on Wikia)
To-Dos: unlike Habits or Dailies, To-Dos don't need to be repeated every day or even on a schedule. You can set a due date, but missing the date or letting the To-Do turn red won't reduce your health. I don't use this kind of tasks, because I prefer to write on my own planner. (Read more on Wikia)
I strongly recommend you to read the HabitRPG Wikia linked above, however if you'd like to use it and you don't know where to start, just set up your Habits, Dailies, To-Dos and keep in mind that completing tasks will allow you to gain experience points (yellow bar) and health (red bar) and committing to bad habits (like my 'extra pizza' one) will make you lose some health points. Along your journey you'll gain money, that will allow you to buy rewards (like armours, weapons and so on), and find eggs containing animals (I recently found that cute pink wolf you can see in my avatar). You can also join some Guilds and start challenges with other people, for example, the Daily 'Read for 15 minutes' is a common challenge I joined being a part of the College Info Geeks Guild.
I started using this game about a month ago and I'm really really happy with it. I committed to read every day, because I actually had a cool purpose (levelling up and purchasing weapons!), and I think it's way more funny checking boxes in a list like that than in an usual minimalist productivity app. Yes, there is also a smartphone/iPhone app for the game and it works well.