Yoshi is the infamous so-called "dinosaur" of Nintendo's Mario series. His first appearance was in Super Mario World for the SNES, where he and the rest of his type of creature were under oppression by Bowser, who apparently trapped them all in eggs. Child Yoshi's were held captive in eight castles all across the world, and on his quest to liberate the Princess, Mario had to conquer each castle belonging to each of Bowser's eight children (were they adopted or did Bowser have some unknown wife?) before being able to take on the big guy himself.
In Super Mario World Yoshi was quite an interesting character. Eating different types of berries would increment one's coin count, and in some special cases, one's time restraint. Yoshi also had the ability to breath fire, fly, and stomp the ground by holding in his mouth a red, blue, or yellow koopa shell respectively. Gamer beware, for Yoshi cannot hold shells in his mouth indefinitely, and after a certain amount of time, Yoshi will indeed swallow the shell.
In special areas of Super Mario World you can find special Yoshis of a variety of colors. Naturally, a blue Yoshi would allow the user to fly by holding in the Yoshi's mouth any color of shell. Red Yoshi lets you breath fire, and Yellow Yoshi allows you to pound the ground. This entails cool combo moves, where a blue Yoshi could possibly hold in its mouth a yellow koopa shell, allowing to both fly and ground pound.
An interesting backstory is revealed in Super Mario World 2, where it is shown that Yoshi knew Mario far before their supposed first encounter. The stork was bringing Mario and Luigi to their parents' house (apparently they are fraternal twins) when Kamek, under the orders of Baby Bowser went to go snatch them. He succeeded in snatching Luigi, but Mario fell towards the island beneath onto the back of Green Yoshi (the storyline here has a charming backdrop tune). Due to some psychic bond between Mario and Luigi, the Yoshis set out to re-unite the two separated brothers.
In this game, the different abilities that depend on color concept was eliminated, and Yoshi was instead equipped with his trademark hovering and throwing eggs created from swallowed enemies. This game also introduced the more distinct Yoshi ground pound, which followed him throughout his gaming career. A cool thing in Super Mario World 2 is that you could get Yoshi high by letting him eat/touch some fuzzy white floating things that look like cotton balls.
Furthermore in his SNES career, Yoshi makes an appearance in Super Mario Kart, where his light weight adds to quick recovery and fast acceleration, with low speed and power.
THE GROUNDBREAKING TRUTH!!!
You may find this shocking, but Yoshi's political alignment is finally revealed in Super Mario RPG. The user arrives at Yoshi's Island to find the inhabitants oppressed by the evil fascist regime of Boshi. Via a race atop Yoshi, the user must use his/her L33T rhythmic button-mashing skills to defeat the dictator Boshi in a one-on-one race and establish a state of anarchy and equality for all Yoshi's!!! (Okay, so maybe I went a little overboard...)
Yoshi made a brief appearance in Super Mario 64 when, upon obtaining all 120 stars, the player could meet Yoshi atop Peach's castle and earn a new jump ability. Also on the N64 Yoshi makes appearances in Super Mario Kart 64 where he plays the same role as in the original Mario Kart, and he also makes an appearance in Mario Golf 64 and Mario Tennis. But Yoshi's next major protagonist role appears in the hit N64 game,
Yoshi's Story (for the N64)!!! The Super Happy Tree was taken by Bowser, the Yoshi population is in a state of depression, and it's up to the optimistic innocent Yoshi youths to get the happy tree with the happy fruit back and save the island. Note that the whole world has been turned into a story book by Bowser, and that after completing each "page", a song with unintelligble words that no one understands is played.
Note that many Yoshi fans were disappointed by the fact that Yoshi's Story was a 2-D side-scroller as opposed to as a fully explorable 3-D world.
On to game mechanics. Yoshi inherits all of his abilities from Super Mario World 2. He eats the baddies, turns them into eggs, does the ground pound, and the hovering in the air thing. To complete each level, or "page", the player must eat forty fruits. To access different stages on the "page", the player must find oversized smiling red hearts.
Yoshi's next playable role is in Super Smash Bros. where he retains all of his trademark abilities. He throws eggs, ground pounds, and eats his opponents (upon which they turn into eggs, which, unfortunately, you cannot pick up and throw). Due to his hovering ability that entails difficulty in aerial control, many find Yoshi to be a challenging character to use.
Yoshi's GameCube debut is in Super Smash Bros. Melee, where he retains all of his abilities from Super Smash Bros.
In the media world, Yoshi did indeed make an appearance in the heinous Super Mario Bros. movie, where he was portrayed as an ugly, monstrous looking thing chained in the Daisy's bedroom in Bowser's castle.
In the toy world, Yoshi is a stuffed toy. They provide a variety of colors of Yoshi for the consumer population, where the most sought-out Yoshi breeds are black and white, the special Yoshi's from Yoshi's Story who could eat anything normally dangerous to eat for the other Yoshi's.
All-in-all, Yoshi kicks ass and his many fans await the next game in which Yoshi is the main protagonist.