There is no difference between these two and you can use them at your will, but there are only a couple of fixed phrases where "till" or "until" are used but they have the same meaning.

There is none. Some claim "till" is the short form of "until" while others claim "till" is actually older than "until." Some claim one is less formal, others claim both are equally formal/informal. The bottom line is there is no difference, they have virtually the same meaning and they are interchangeable.

The word "until" is slightly more formal than "till", but otherwise, they have exactly the same meaning. Do keep in mind, a "till" is also the place where the money is kept in a shop, also know as a cash register.