There are many items in Super Mario World. Most of these items can benefit you in some way.
Coins
There are several types of coins in Super Mario World. Coins can help you gain extra lives, so collect coins whenever possible.
The following types of coins exist:
- regular coins
- Dragon Coins
- monster coins
Regular coins are the most common. Unlike previous Mario games, you normally do not gain points for collecting them unless you get them from special blocks. If you turn an enemy into coins by fireballs or by reaching the goal, you can gain points from the coins. Also, if you get coins by jumping on a yellow Koopa, you also get points. You can gain an extra life per every 100 regular coins.
Most levels have exactly five dragon coins. On any particular level, you will gain 1,000 points for the first dragon coin, 2,000 points for the second, 4,000 points for the third, and points 8,000 for the fourth. Any additional dragon coin you collect will result in an extra life. Once you collect five dragon coins from a level, you cannot collect them again unless you reset the game. Dragon coins also count as regular coins. If you have 99 regular coins in your counter, and you take the fifth dragon coin, you will gain two extra lives.
The gray-colored P-Switches will turn certain enemies into silver coins temporarily. For every coin you collect before the time expires, you gain an increasing number of points per coin, until you reach 8,000. The next coins will earn you an extra life, two extra lives, and finally, three extra lives for every coin you collect.
Power-ups
There are four power-up in Super Mario World, Super Mushrooms, Fire Flowers, Cape Feathers and stars. Power-ups can be found in some yellow blocks, ? blocks, hidden blocks and note blocks. Glass blocks will always contain an item that transform from a Super Mushroom to a Fire Flower to a Cape Feather to a star and then repeats the cycle forever. Green ! blocks will always contain a Cape Feather. Yellow ! blocks will always contain a Super Mushroom. Power-ups can also be found on on their own (i.e. not in blocks) in parts of certain levels, and enemies and items Mario carries across the goal post may transform into powerups.
Super Mushrooms turn small Mario into Super Mario and make Mario bigger. Mushrooms with give Mario 2 hit points (if he is hit he turns to small mario, if he is hit again as small mario he dies. There is also a green mushroom which when collected will give you a life. Fire Flowers turn Mario in Fire Mario. Cape Feathers turn Mario into Caped Mario.
Stars make Mario invincible to enemy hits for a short period of time (he may still die by falling, time running out or being squished). During this period, most enemies Mario touches will die instantly. The first enemy Mario kills with a star will give him 200 points, the second 400, the third 800, then 1000, 2000, 4000, 8000. Every enemy killed afterwards (after the seventh) will be a 1-up or a 2-up. This is one of the quickest ways in the game to get additional lives.
Eggs
Certain blocks will contain an egg. An adult Yoshi will instantly hatch out of these eggs. If egg is found on its own (i.e. not in a block), a baby Yoshi (not an adult) will hatch out of it. If you already have a Yoshi, you will get a 1-up mushroom instead.
Crescent Moon
You are able to get tiny smiling crescent moons which are quite rare. They give you three lives if you collect them. One is located in Yoshi's Island 1 and is explained in detail.
Shells
Koopas have special swallow and spit properties for Yoshi, depending on the color of the shell.
When holding a shell in his mouth, Yoshi has only 25 seconds before he is forced to swallow it. He'll start to gag, and you'll see a lump in his throat like he's about to swallow, and once the time is up he'll eat it. However, there is a way around this if you're holding any other shell than a Red Shell - if you spit it out where you know it will ricochet back at you, you can just pick it up again and reset the 25 second timer.
If you pick up a red shell or a shiny shell, when you spit it the shell will be transformed into fire balls - knowing this, you'll need to use the shell effectively or else it will be wasted. There's a trick with these shells if you somehow don't hold Y or X after putting them in your mouth (pressing the button again will obviously spit it out) - simply press 
, then hold the button you need for running, and then press again.
Red Shell
If Yoshi eats a red shelled Koopa or just the shell itself and then spits it out, three fireballs will fire from his mouth.
Blue Shell
If a blue shell is eaten, Yoshi will sprout wings and be given unlimited flight until swallowed or spit out - you can either tap or hold 
. Yoshi will spit this out as normal.
Green Shell
No properties are gained; Yoshi will spit this out as normal.
Yellow Shell
The stomp is gained - this ability adds an area of effect attack whenever Yoshi jumps and hits the ground (you'll know it's working because a loud thump sounds as well). Two small clouds will appear to either side of him, protecting him from danger and knocking his enemies over. This is the same effect as when Mario dives using the cape. Yoshi will spit out yellow shells as normal.
Shiny (Tornado) Shell
When a shell-less Koopa Troopa hops into a yellow shell, it spins and spins and chases Mario! You can't defeat it with anything other than another object or swallowing it with Yoshi. When Yoshi puts it in his mouth, he will sprout wings, gain the stomp effect, and fireballs when spit out (thus, it can only be used temporarily). In the Green Switch Palace, you can use him to take down Koopas and get a 1UP (or simply use the switches appropriately).