unto is a preposition meaning 'to' or 'until,' typically used in formal contexts and often found in religious texts. Its usage is similar to that of 'towards' or 'as far as.'
1. Woe unto them that are wise in their own eyes, and prudent in their own sight!
2. Moses gave unto them, even to the children of Israel, a charge, saying, Take ye from among you an offering.
3. Not unto us, O Lord, not unto us, but unto thy name give glory.
note that you can often substitute the word 'to' for 'unto' in informal contexts. Commonly, people mistakenly use 'into' when 'unto' should be used; for example, 'Give unto me' would be the correct usage, rather than 'Give into me.'