Yoshi is a dinosaur who appears in various colors (from Green to White) from the Mario series. In nearly all of his games, he possesses the ability to swallow enemies, using his tongue to catch them. In Super Mario World 2: Yoshi's Island, he is seen making eggs from his enemies he has swallowed. He is a formidable ally of Mario who can hop on Yoshi's back to ride faster. Yoshi is set to appear in two of his own upcoming games, including Yarn Yoshi (tenative title) on the Wii U and Yoshi's New Island on the 3DS.
Yoshi first appeared in Super Mario World as one of the main characters. When Peach was kidnapped shortly after the start of her, Mario, and Luigi's vacation on Dinosaur Island, the Mario Bros. found a Yoshi Egg, with Yoshi emerging from it and helping the bros. on their adventure. He tells them that Bowser had kidnapped the other Yoshis and trapped them in castles, meaning that they had to saved as well. In gameplay, some ? blocks can contain Yoshi Eggs, which make Yoshi appear. Mario must jump on Yoshi's back to ride him. Yoshi can do several things that Mario can't, including being able to jump on some spiky enemies without being harmed. Most notably, Yoshi can swallow enemies and turn them into eggs, which can contain items or a 1-up. However, Yoshi cannot be used in fortress, castle, and ghost house levels (with the exception of the Sunken Ghost Ship).
Yoshi can be used from level to level, as Mario can ride on him in the map screen. Most of the more recent games don't have this feature. This means SMW is also the only Mario game where Yoshi can enter certain levels, such as underwater ones.
After all of the other Yoshis are saved, they are returned to Yoshi's House, a "level" in the first world of the game (Yoshi's Island).