Koopa Troopas are turtle-like creatures. They are the most common classification of Koopa and are usually thug-like. Koopa Troopas are the standard troopers of the Koopa Army who work for Bowser. As the lowest ranking members of the Koopa Army, they just patrol their assigned areas on foot. Koopa Troopas first appeared in the arcade game Mario Bros. under the name "Shellcreepers" as enemies that have to be flipped. In Super Mario Bros., they were renamed "Koopa Troopas". They can be used as projectiles after they are stomped on and retreat into their shells. Also, green-shelled Koopa Troopas will walk off of platforms (even to their death), yet red-shelled Koopa Troopas change direction at the end of a platform. Super Mario World brought drastic changes to the Koopa Troopa, altering them to walk on two legs and wear shoes. They can also be ejected from their shells, revealing them to be wearing an undershirt. In Super Mario Galaxy, they reverted back to their quadrupedal movement.
In most games there are three types of Koopa Troopas: Green and Red Koopas, and Paratroopas. Green Koopas walk in a straight line and mindlessly fall of ledges. Red Koopas will also walk in straight lines but avoid falling off the edges of platforms. Paratroopas can have red or green shells and have wings. In the SMB series, green ones bounce along the ground (with the exception of level 7-3, where they fly horizontally), while red ones follow a set path (up and down). They fly left or right or jump up and down in Super Mario World. Koopa Troopas were the most regularly-seen minions of Bowser in The Super Mario Bros. Super Show!, where they were voiced by John Stocker.
The Paper Mario series introduces friendly Koopas that have their own towns, and are even rescued by Mario on occasions, as well as evil Koopa Troopas that wear sunglasses. Generic, friendly Koopa Troopas have also appeared throughout the Mario Party series as NPCs. Koopa Troopa is a playable character in some games of the Mario Kart series (where their shells can be used as weapons) and in various other Mario Sports games along with Paratroopas.
In Super Mario Galaxy, Koopa Troopa shells are used as vehicles underwater. Red Koopa shells are fast, while green shells are slow. Red lights are noticable in the back feet 'holes' when slowing down, similar to those on cars.
One Koopa in particular, named "Koopa the Quick", is an ally of Mario in Super Mario 64, usually challenging him to a race, too proud to believe that Mario can beat him, and if he can, he becomes frustrated but hands over the star he promised anyway.