Flipnote Studio is an application distributed for free with the Nintendo D Si. Flipnote Studio allows you to create simple hand-drawn animations. You can then add sound to these animations, if you wish. The true entertainment value of Flipnote Studio, however, comes in the accompanying Internet community, Flipnote Hatena. Hatena is where you share the flipnotes you've made. On Hatena, you can watch other people's flipnotes, download them, and edit them (if the original creator gives you the go-ahead). If people watch your flipnotes and like them, they may give you "Stars", which simply serve as a means of quantifying how much people like them. There are five different types of Stars: gold, green, red, blue, and purple, from most common to least common.
There is also a way to access Hatena from the Internet. Specifically, it can be found here.