This is Mario's weakest form, but it is the standard form used whenever a new game starts. If Mario is in one of his upgraded forms and takes too much damage, he will return to this state. While in this state, if Mario takes damage once, he will lose a life.
This is Mario in his full size after using a Super Mushroom. Mario gains the ability to break Brick Blocks in this state (which he will not lose even if upgraded further), and additionally he can take one extra hit before losing a life.
After utilizing a Fire Flower Mario will turn into Fire Mario, giving him the ability to throw fireballs at enemies. He will not turn to this if he is Small Mario, in which case he will turn to Super Mario (international release only).
After getting a Starman, Mario will become invincible, being unable to be harmed by any enemies and most obstacles and getting squished. Along with the bonus of invincibility, Mario can also defeat most enemies without jumping on or throwing projectiles at them.