Princess Peach, or just Peach for short, is the princess of the Mushroom Kingdom and made her first appearance in the original Super Mario Bros. She is typically the damsel-in-distress, as Bowser constantly kidnaps her, although she had a couple of roles outside of that. Super Mario Bro. 2 marked her first appearance as a playable character in a mainline Mario game and Super Mario 3D World marks her first appearance since then, although she is playable in a bunch of spinoff games.
Peach is a character that might be more adept for beginners to the game (or platformers, in general), as she has the ability to float for a short distance after making a jump. This basically gives her a longer jump than any other character in the game, giving her an advantage in some levels. However, because of her longer jump, her speed is the slowest out of the entire group of playable characters.