Mega Man Zero (originally known as Rockman Zero) is a continuation of the X series but with a new drawing style and some new variations on the game play.
The series began on the Game Boy Advance, coming out of the gate with polished play control and detailed but colorful graphics. The story starts 100 years after the X series, featuring the same X and Zero but with totally new appearances. Here, as the title implies, the series focuses more on Zero than X, and Zero is the only playable character.
The Mega Man Zero games were designed to be somewhat more difficult than games in the classic and X series; there are fewer 1-Ups (continues) and the player often must gain high ranks by performing well in order to acquire new abilities.