It's an extremely annoying and lazy way to say "thank you".

I guess "ty" stands for "thank you", but don't assume this is universally understood or accepted. Different people will have their own preference. And James is right, it's pretty lazy!! Perhaps OK for sms and chat but in an email it looks sloppy. Other variants are: "thx", "tq".

yes it's "thank you"

is it "Thank you "?

ty = thank you