'What you up to' vs 'what are you up to'

1 What you up to?
2 What are you up to?
Any difference between them? What are their definitions exactly?

The difference is that the first sentence is not good English. In spoken English the short form ‘What’re you…’ does sound a bit like ‘What you…’, which may confuse learners.

‘What are you up to?’ means ‘What are you doing?’.