Vocaloid. It's kinda hard to explain. Vocaloid itself is a program that you can make music with. They record every sound a language can make with a voice actress/actor, and then sell the program where users can make songs with the program. Vocaloid characters are the anime-styled characters that were created to be a representation of the voice (My avatar is the most famous one, Hatsune Miku) I love their characters designs, and a lot of the songs are catchy :)
Yes, I have played X but not Y. It was pretty good, but I wouldn't worry too much if you don't have them yet because they'll always be there :)