Bowser becomes bigger and more powerful than his normal size. Giga Bowser can take damage, but will not flinch, which may let Bowser get KO'd when he reverts to normal Bowser. however, like many Final Forms, it is not impossible to get KO'd with such a lumbering and slow form.
Giga Bowser first appeared in Super Smash Bros. Melee as a hidden boss in Adventure Mode. To get to him, Adventure Mode must be done on Normal difficulty or higher, in 18 minutes or less, and with no continues. Giga Bowser also appeared in the final event in Melee as an opponent, alongside with Ganondorf and Mewtwo. Giga Bowser would eventually return in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS and Wii U, but this time as a semi-playable character in Bowser's Final Smash. He reprises the same role in SSF2.
Strictly speaking, Giga Bowser did not appear in any Mario games, despite being a Mario character. Giga Bowser is likely based on the tradition of final boss fights in Mario games involving Bowser growing to gigantic size; this first appeared in Super Mario World 2: Yoshi's Island, featuring a massive version of Baby Bowser.