Hi Magia,
Yannis is a very common name in Greece, and Yani is used as a nickname. The name is the Greek
John.
Yanakis is very indicative of a surname. The "akis" ending is very popular surname ending among people from Crete. I was told the "akis" ending was related to the Greek work Pathakyas (phonetically spelling), which means children. So that surname would mean 'children of
John'.
Pavlos, will come to comfirm/correct. :-P