Pichu is an electric-type Pokemon from the Pokemon series of video games that looks like a small yellow mouse who is the baby form of Pikachu. The fact it is a baby Pokemon means it can be only be obtained by breeding(an exception to this rule being Mirage Island of Ruby, Sapphire, and Emerald, a hard to reach place where the only wild Pokemon are Wynaut, the baby form of Wobbuffet). Specifically, you need to breed eighter two Pikachu/Raichu; one of each gender, a female Pikachu/Raichu and a male Pokemon that can breed with it, or a Pikachu/Raichu and Ditto to get an egg containing a Pichu. The games you can get Pichu in are every handheld game ever since its debut games, Gold and Silver.
Pichu is not skilled at storing its electricity, so it will often shock itself while amused or startled. This is shown in the Nintendo fighting game Super Smash Brothers Melee, as Pichu recieves a little damage whenever it uses an electric attack on said game.
Pichu is often the focus of special moves in the video games. For example, in Pokemon Box, a storage system for Pokemon, if the player manages to store 1499 Pokemon in the game, the final space of the box will be filled: A Pichu that knows the move Surf, which it cannot normally learn, despite the fact many other Pokemon can learn it by HM, mostly those of the water type. Another example of this is if the player breeds two Pikachu/Raichu, with one holding a Light Ball, the Pichu that hatches will know the move Volt Tackle, which normally can't be learned by any Pokemon.