Super Mario Galaxy contains a whole mess of different abilities that Mario can use to help him gather stars, or just for fun!
Mushrooms' effects vary depending on their appearance. Some help you gain a life, others help you gain an ability.
 Bee Mushroom
A Bee Mushroom is yellow with black stripes (like a bee), and will turn Mario into Bee Mario once one is collected. As Bee Mario, by pressing (and holding) A, you can fly until the fly meter goes all of the way down to zero. Once you land, it will replenish. To lose Bee Mario take damage or touch water.
 Boo Mushroom
By gathering a Boo Mushroom, you will be transformed into Boo Mario. As Boo Mario, you can do what you would normally do to spin, in order to turn invisible for a few seconds, enough time to go through walls. By pressing A, you will float up. You will have this ability until you get hurt or run into light.
 Super Mushroom
A Super Mushroom is red with stars on it. It will double Mario's lifespan of 3, to 6. Once you've lost three bars of health, returned to the observatory, or die, it will disappear. You can't have two of these at the same time to double the health meter again unfortunately.
There are two types of flowers in the game that are power-ups, a fire flower and an ice flower.
 Fire Flower
A red flower that when grabbed, will make Mario turn into Fire Mario which will have him wear classic red and white. By shaking the Wiimote or Nunchuck, Mario will spin but will also throw a fire ball out. This will run out after a certain amount of time.
 Ice Flower
A blue flower that when collected, Mario will turn an ice-ish blue. Any water that he touches, even under his feet, will instantly freeze and stay frozen for a small period of time. After a certain amount of time the ability will wear off.
Like the flowers, there are only two types of power-up stars to get in this game, but they are a lot rarer than other items.
This is a flashing, rainbow star that once in your possession, you will be invincible for a short amount of time towards other enemies, and you can run faster too. You will be flashing rainbowy for the short amount of time you have it.
 Red Starman
This is simply a starman, but red. When grabbing this, Mario will turn black and red and have the ability to fly. Simply be in mid-air and spin to fly where you want to go. It will run out after a pretty descent amount of time.