By driving through an 
, you'll get an item that will help you - or hinder your rivals - during the race. Some items are more useful than others, and the further up the field you are the less powerful items you'll recieve.
Mushrooms
Mushrooms give you a speed boost.
Mushroom
A single-use mushroom to give your racer a temporary speed boost.
Triple Mushrooms
Three mushrooms together, to give you three separate boosts.
Golden Mushroom
Gives you a potentially infinite number of speed boosts, but only stays in your item slot for a short time after the initial use.
Mega Mushroom
Increases the size of your racer and their vehicle temporarily, increasing their speed and flattening any other racers you drive into or over.
Shells
Shells are offensive weapons, fired at other racers or just dropped behind you as an added obstacle for your rivals.
Green Shell
Travels in a straight line after being fired, and bounces off walls for a short while before it breaks.
Triple Green Shells
Three Green Shells. Pressing the Use Item button once will cause the shells to start orbiting your racer, forming a protective shield around the vehicle against other racers and most items.
Red Shell
Travels along the track, homing in on the person in front of you. Can be blocked using a Banana, or avoided on some tight corners.
Triple Red Shells
Three Red Shells. Pressing the Use Item button once will cause the shells to start orbiting your racer, forming a protective shield around the vehicle against other racers and most items.
Blue Shell
An advanced homing weapon, that flies over the track and takes out the race leader - as well as anyone else in the vicinity. The Blue Shell cannot be blocked or avoided, unless the target has an activated Star.
It's not advisable to fire this weapon if you are in first place.
Dropped Items
These items are dropped behind your vehicle as obstacles to other racers, but can also be fired ahead using 
, or holding 
, 
or 
while pressing the Use Item button.
Banana
Causes anyone who runs over it to spin out of control for a few seconds, and can block Red Shells.
Triple Bananas
Three bananas. Pressing the Use Item button once will cause the Bananas to trail behind your kart, protecting you from incoming Red Shells; press the button again to use a Banana normally.
Bob-Bomb
Explodes, causing anyone in the vicinity to lose control for a few seconds.
Fake Item Box
From a distance, looks like a regular Item Box, but will cause any racer who drives into it to lose control for a few seconds.
Misc.
Bullet Bill
Pulls you very fast along the racing line for a short period. Any other racers in your path will be flattened or thrown out of control.
Star
Makes your racer invincible for a short period, and increases the maximum speed of your vehicle for the duration. The Star also allows you to drive off-road without suffering any slowdown.
Blooper
Sprays ink across the screens of your opponents, temporarily preventing them from seeing the track clearly. Can be cleared off faster than normal by using a Mushroom boost.
POW Block
Causes a quake that bumps all other racers off the ground, causing them to lose control temporarily. Can be avoided if you're jumping at the moment of impact.
Thunder Cloud
When activated, the Thunder Cloud hovers above your racer, and after a short period fires a lightning bolt that shrinks you. Can be passed onto other racers by bumping into them.
Lightning
Fires a lightning bolt on all other racers, shrinking them and reducing their speed. If a racer is in the air, or over a jump at the time, they will fall straight down.