The Koopas are Turtles, which have, depending on their color, different skills. Mario can knock them out and use them as a weapon. They come out of their shell if you don’t use them. But in some Games, they are allies of Mario.
Goombas are the most famous opponents of Mario. They are small and brown, and their appearance is based on mushrooms. In 2D-games, they run from the left to the right, and in 3D-games, for example Super Mario 64, they run to Mario, if they see him.

Bomb-ombs are opponents, which also run up to Mario and explode after that. If Mario jumps on them, he could make them harmless. In some games, he could pick them up and throw them. There are also kindness Bomb-ombs, for example in Super Mario 64, there are pink Bomb-ombs which are helping you in the game.
Cheep-Cheeps are Fishes, which are living under the water, but sometimes they are also above the water. They jump fast to the water surface. There are also some other Cheep-Cheeps, for example the Deep-Cheep, who chases Mario, when he saw him, and the bigger Cheep Chomp, who tries to eat Mario.
The Hammer-Brother is a rare opponent of Mario. They try to throw their Hammer at Mario and jump while away from Mario. Mario can knock them out, if he jump on their head. There is also a Boomerang-Brother, which has the same principal as the Hammer-Brother, only that his weapon is a boomerang.

The big canon, is a canon, who is not a direct opponent from Mario. They are shooted out of big holes, and Mario only has to get out of their way.
The Watch-Dog is a dog on a chain, and if you go close to him, he tries to catch you. In most games, you can rescue him, by jumping on the pflock.