Mushrooms are different items in New Super Mario Bros. There are 4 different types.
Super Mushrooms are power-ups in the game. It is a red mushroom with white spots. When Mario touches this, he grows.
1-Up Mushrooms are very special power-ups. When Mario gets one, he gets an extra life. They look exactly like Super Mushrooms, but they are green mushrooms with white spots instead of red ones.
Mini Mushrooms are power-ups. They are dangerous, as taking a hit from an enemy causes Mario to lose a life. You can walk across water, however if it is too high and you stop walking you will sink. Dashing is highly effective. Mario can also enter mini pipes. It appears as a very small blue mushroom with white spots.
Mega Mushrooms are almost as rare as 1-Up Mushrooms. When Mario gets one, he turns huge and almost covers the screen and can smash through almost anything, and even damage blocks that not even Broozers can and damage pipes, the flagpole, and enemies.