Description of the main characters
He's Nintendo's mascot and usually the main hero of the games in which he appears. In 1981 he has his first appearance in the NES-game "Donkey Kong" in which he tries to save Pauline from DK. Mario is actually a plumber but most of the time he's not repairing pipes but saving the world and rescuing a princess. Nintendo doesn't tell us a lot of Mario's character. Contrary to some of his friends or enemies, his personality is rather unknown and boring, Mario being a standard hero. His most famous games are "Super Mario Bros." (NES), "Super Mario Bros. 3" (NES), "Super Mario World" (SNES), "Super Mario Land" (GB), "Super Mario 64" (N64), "Super Mario Galaxy" (Wii) and "New Super Mario Bros." (NDS). He's also the star of the popular Mario Kart- and Mario Party-games.
Luigi is Mario's rather anxious younger brother and also a plumber. Usually he's only a minor character, standing clearly in the shadows of his brother. In older games like "Mario Bros." (NES, Luigi's first appearance) or the Super Mario Bros.-series it's possible to control him but he doesn't really have an own personality, being rather a green copy of Mario than an independent character. In some games like "Super Mario World" (SNES) he only appears in the multiplayer-mode. His most important appearances are in the "Mario & Luigi"-RPGs (where he's in a funny way sometimes rather an obstacle than a big help for his brother) and in "Luigi's Mansion" (GC), the only game in which he's really the main character.
He's Princess Peach's closest servant and a friend of Mario. Many people of the kingdom look like him and the inhabitants are sometimes also called Toads. The little Mushroom guy usually plays a rather small role in Nintendo's games, accompanying and helping Mario. "Super Mario Bros. 2" (NES) is the only platform-game where he's a playable character. However, he also appears in every Mario Kart and in most Mario Partys, in some games also as a kind of referee or host (for example in the first Mario Party). His sister Toadette makes her first appearance in "Mario Kart : Double Dash" (GC).
Yoshis are friendly dinosaurs who decided to ally with Mario in "Super Mario World" (SNES) after being terrorized by Bowser. They are very helpful as they can eat most enemies and as Mario can ride on them. There are many Yoshis, having different colors (the green one is the most famous Yoshi), who live on their own island. As Yoshi became very popular, he was soon given the role as main hero in "Super Mario World 2 : Yoshi's Island" (SNES) where he helps Baby Mario against Kamek and Baby Bowser. The green dino has also major appearances in "Yoshi's Story" (N64), "Yoshi's Universal Gravitation" (GBA) and "Yoshi's Island DS" (NDS). Beside this, he also appears in many Mario-games, for example in "Super Mario RPG" (SNES), "Super Mario 64 DS" (NDS), "Super Mario Galaxy 2" (Wii) and in most Mario Kart-, Mario Party- and sport-games.
Peach Toadstool is the princess of the Mushroom Kingdom and often occupies the role as damsel-in-distress. She's frequently in trouble as Bowser likes to kidnap her (for example in her debut appearance in "Super Mario Bros."), in later games it's even obvious that he's fallen in love with her (for example he wants to marry her in "Super Paper Mario"). However, the princess doesn't like the evil Koopa-king and she's rather interested in Mario, even though she's usually a bit reserved. Peach participates in most Mario Kart-races and sport competitions. She plays an important role in "Super Mario Bros. 2" (SNES) and "Super Princess Peach" (NDS - where it's her turn to save her friends), the two only platform-games where she's a controllable character.
She's the princess of Sarasaland and a friend of Peach. She got kidnapped in "Super Mario Land" (GB) by the alien Tatanga. Afterwards, she wasn't seen for a long time before reappearing more and more in Mario's spin-offs. She usually only plays a minor role, though. There are some allusions that she's in love with Luigi, for example there is a statue in "Mario Kart Wii" showing them dancing.
Bowser is the main antagonist in Mario's world and usually the final boss in his games. He's a giant turtle with a spiky shell and he's usually able to breathe fire. Bowser is the king of the Koopas but he wants to rule the entire world (or at least the Mushroom Kingdom). That's why he often attacks Mario's country and frequently kidnaps Princess Peach. It's by the way often suggested that he likes her (in fact she's probably the only person he cares for). Bowser is usually arrogant and ruthless. He doesn't seem to be very intelligent but he's strong and generally taller than Mario. He appears in many games as the main villain, such as "Super Mario Bros." (his debut), "Super Mario Bros. 3" (with his 7 children, the Koopalings), "Super Mario World", "Super Mario World 2 : Yoshi's Island" (as Baby Bowser), "Super Mario 64", "New Super Mario Bros." (NDS) and "Super Mario Galaxy" (Wii). He also appears in most of Mario's spin-offs, either as a villain (in most Mario Partys) or as a playable character (in the Mario Kart- and in most Super Smash Bros.-games). Being usually Mario's archenemy, he exceptionally helps the plumber in "Super Mario RPG" (SNES) and "Mario & Luigi : Superstar Saga" (GBA) (even though it's for egoistic reasons).
These turtle-like creatures usually have a coloured shell (typically red or green) which can be used as a projectile to attack other enemies. They are (beside the Goombas) the most common enemies in Mario's platform-games after their debut in "Super Mario Bros." (NES). The Koopas are a group of many different species, the best known are the Koopa Troopas (who just walk around) and the Koopa Paratroopas (also called Parakoopas, they are able to fly). The word "Troopa" can be seen as an indication that they serve Bowser's army. However, not all the Koopas are in the hostile troop. In some games, for example in "Super Mario 64" and the Paper Mario-parts, you can find peaceful Koopas, e.g. Kooper who joins Mario's party to help him in "Paper Mario" (N64). Beside their normal appearances in many platform-levels as common foes, the Koopas are playable characters in some Mario Kart- and Mario Sports-titles.
Goombas are very common enemies but as they are very small and quite slow, they don't present a real danger for Mario. They can be easily defeated, usually with a single stomp. They resemble brown mushrooms (except for "Super Mario World" where they are red and spheric) and it's often suggested that they are not very intelligent. Goombas can be seen in most games, without playing an important role, though. They used to live peacefully in the Mushroom Kingdom but a large majority of them became traitors and joined Bowser's army. However, Mario also meets some Goombas in the RPGs who work as independent thieves and some Goombas who help him, for example Goombario in "Paper Mario" (N64) and Goombella in "Paper Mario : The Thousand-Year Door" (GC). By the way : you'll also meet more and more (hostile) giant Goombas in newer games like "Super Mario Galaxy" (Wii) or "New Super Mario Bros." (NDS).
(more characters are following soon)
Finally you can see on this picture mady by Taylor Southerland for mariomonsters.com most enemies and characters in action :