The tin whistle is closely related to the flageolet and is often referred to as such (mainly by music stores that consider "tin whistle" or "penny whistle" to be too low class). They are the same in that they have a mouthpiece which is narrower and flatter than that of the recorder, giving them a much more piercing sound. The chief difference between a tin whistle and a classic flageolet (which was actually an orchestral instrument back in late Rennaissance and early Baroque times) is that, while the latter can have many holes and even keys like a modern woodwind instrument, the former has only six holes. This makes the tin whistle much easier to play. There is also the matter of the tin whistle being made of, well, tin, whereas a flageolet is usually wood (or, in the case of modern, toy ones, plastic).
Tin whistles come in many different keys. The traditional one is in D. I have three of those. I also have a C one and a little, tiny F one. The D one seems to be the most comfortable for me, as the fingerings actually match that of a soprano recorder without the low C hole. It is also the easiest to control. I can play three full octaves comfortably on a D whistle. Almost all of my tin whistle recordings and performances, professional and amateur, were done using one of my D models.