Τι means what and ποιος/ποια/ποιο all mean which (note that ποιος and ποια can also mean who). However, in English the word what is widely used, while in Greek ποιος/ποια/ποιο is more common. Τι isn't declined while ποιος/ποια/ποιο are declined.
Τι έφαγες; = What did you eat?
Τι είναι αυτό; = What is this?
Ποιο είναι το όνομά σου; = What is your name?
Ποια είναι η λύση; = Which is the solution? (or what's the solution?)
I hope that this helps. :-)