You can collect many different Powerup items by jumping into question mark boxes or finding secret areas where they hide. They help you beating many levels and you can save one Powerup for later in the Itembox on the top of the Screen. If Maryo got an Powerup and gets hit by an Enemy he doesn't die but becomes small Maryo again but be careful not to fall out of the level which is always deadly. The list below explains the various Powerups you can find.
Transform into Big Maryo who can get hit once but then transforms back to small Maryo. Superior Powerups often only pop up if you already got the Red Mushroom.
Transform into Fire Maryo who can shoot Fireballs. Many enemies can be hit with Fireballs but some are immune like the Thromp, Rokko and Lava Gee.
Transform into Ice Maryo who can shoot Iceballs. Most enemies can be hit with Iceballs also some who where immunse to Fireballs but some not like the Rokko and End Bosses.
Transform into Ghost Maryo for a short time who is hidden to all Enemies but still can get hit. Enemies who only react if Maryo is visible like the Rokko, Thromp and Spika will not chase you anymore but beware the Piranha Plant which now always jump out. Additionally you can now see hidden Ghost Blocks who can take you to hidden areas and valuable items.
Gives you the Star Power for some time which makes you invincible and lets you hit enemies on contact. Additionally you can run faster, jump higher and access areas normally unreachable by walking on enemies as Spika, Eato and Rokko.
Gives you one extra Life. You also get one life up after collecting 100 Goldpieces. Lookout for these!
Gives you 3 extra Lifes ! A secret and often hidden item :)
Be careful not to get the Posion Mushroom or you will downgrade or even die if you are small Maryo.