Till is not a shortened form of until. Till has actually been in use longer than until.

They are often interchangeable; they both mean ‘up to’ (a point in time or event).

However, until is more likely to be used at the beginning of a sentence while till often appears before a noun. Until is also slightly more formal.

The contraction of until is ‘til and many sources will tell you not to use it. You can ignore them in informal contexts.