This overgrown Koopa Troopa of a tyrant is always up to something, and that "something" usually involves kidnapping the princess and trying to take over the Mushroom Kingdom
 
Introduction
    Even though Mario always beats him he just keeps on trying. What a dork. On the other hand if Bowser gets tired of kidnapping Peach then it wouldn´t be anymore fab Mario-games so what the heck, Go! Bowser Go!

Koopa in his Koopa-copter from SMW

    It all started out on the NES with the Super Mario Bros series and continued on to the SNES with Super Mario World. Then it was quiet from Bowser except for some smaller appearances. I guess Bowser took a break from trying to kidnap Peach because he appeared in both Super Mario Kart and Mario's Time Machine, where he tried to open the biggest museum ever by traveling to different times in history and steal improtant artifacts. Of course Mario spoiled his plans for that one too... In Mario is Missing he tried a new approach. To kidnap Mario, instead of Peach. It nearly worked but Luigi came and saved the day.
    Bowser has also made some some brief appearances in the Game 'n' Watch Gallery games. In Game 'n' Watch Gallery Bowser appears in the redesigned version of Oil Panic and in Game 'n' Watch Gallery 3 he makes an appearance in Mario Bros.
 
Kids say the darnest things...

    Did you think Bowser was sweater as a child? Forget it, he was already as short tempered and ugly as a child, just look at him in Yoshi's Island. Stomping Kamek flat as a leaf just for waking him up and pushes Mario of Yoshi's back because "What kind of gween donkey is that? Looks wyke fun! Me wanna ri-ide!!".
   Maybe you assumed that Bowser had learned himself a lesson, but no. In Yoshi's Story he stole the Yoshi's Super Happy Tree in an attempt to get rid of the Yoshi's once and for all.
 

Weren't they arch enemies?

    The first family member of Bowser's ever seen was not in the Mario video games, instead, it was in The Super Mario Bros. Super Show! episode "Do You Princess Toadstool Take This Koopa...?". This episode introduced Mama Koopa, King Koopa's (as Bowser was called The Super Mario Bros. Super Show!) mother, who was a bossy and bullying woman. Additionally, King Koopa (as Kid Koopa) also mentioned a grandfather named Poopa La Koopa in an earlier episode of The Super Mario Bros. Super Show! entitled "Butch Mario & the Luigi Kid", with character's apparent motto being "Cheat, beat, and be merry!"

 

 

The 7 Koopa Kids of Bowser

The game Super Mario Bros. 3 introduced the first seven of Bowser's eight children, the Koopalings; who, after their first appearance in Super Mario Bros. 3, would go on to appear in several other games with their father, as well as in various other Mario-related medias, such as cartoons and publications.

In issues of Nintendo Comics System, Bowser would occasionally mention his father, who is heavily implied to be a criminal currently being chased by the authorities. Bowser and his children were also shown to have a pet, Pookie, in Nintendo Comics System, who was a rabbit who was often abused by his owners. Pookie eventually escaped from Bowser's Castle in the comic "Bowser Knows Best".

The Super Mario Bros. movie, two idiotic cousins of King Koopa, named Iggy and Spike, are introduced. Despite the presence of cousins though, a brother or sister of King Koopa is never mentioned in the Super Mario Bros. movie. Also, King Koopa's second-in-command in the Super Mario Bros. movie, Lena, is implied to have some sort of relationship with King Koopa. Yoshi was also portrayed as the pet of King Koopa, though the two seemed to have hatred for one another, with Yoshi, much like Pookie before him, suffering much abuse at the hands of King Koopa, who referred to the non-humanoid dinosaur as a "throwback".

Super Mario World 2: Yoshi's Island introduced Kamek, a Magikoopa who is seemingly the caretaker and possible adoptive father of Baby Bowser, often shown to take care of him. The Mario Party series also introduced Koopa Kid who, despite having an uncanny resemblance to Bowser, seems unrelated to him; though Bowser acts somewhat like a father to Koopa Kid, doing things such as grounding Koopa Kid or banning him from playing with his toys when he fails.

The game Super Mario Sunshine introduced Bowser Jr., Bowser's eighth and youngest child who, unlike the Koopalings, resembles him greatly.

In Super Paper Mario, Bowser and Princess Peach are forcibly wedded by the sinister Count Bleck and Nastasia. Though Bowser regards this wedding as official and Princess Peach as his wife, Princess Peach herself cites the wedding as a sham and disregards it as official.

The UK Nintendo Power once said that Bowser had a wife named Clawdia Koopa; however, as Clawdia is never referenced to again, neither in any games or later issues of Nintendo Power UK, it is often believed that Clawdia was simply a joke.

 

Some credit goes to the Super Mario Wiki for information.