" 'til " is just a contraction of " until ". To me, it seems like "until" is the stressed form of the word and " 'til " is the unstressed form. " 'til " is also used more often in spoken English, whereas " until " is used in writing, especially formal writing.

Unless you are reading a paper written by a classic literature student, you will likely never come across the word "till" in written form unless they are talking about a cashier (as in "I took my groceries to the till"). More likely you will see the shortened form of "until" which is " 'til " (notice the appostrophe at the beginning, and only 1 L)... At least this applies in Canada. :)

The two words can be used as prepositions and conjunctions. They mean exactly the same. 'till' is informal 'until' (note just 1 l) is more formal.

You can use them interchangeably, but "until" has only one L.