Arrancar

Put simply, an Arrancar is a hollow who obtained the powers of a Soul Reaper. Depending on the level of power there is a higher possiblity of taking a humanly shape. Arrancar's appearance range from seeming like a regular hollow to the partial mask,zanpakuto, and a white shuhakusho(soul reaper uniform.)The more powerful/intelligent a hollow the more likely it will be to achieve a human-like form when becoming a arrancar. Only vasto lorde type hollows are gaurenteed to become human-like.

A Arrancar is the opposite of the vizard ( shinigami that have obtained hollow powers) whereas arrancar are hollows who have obtained shinigami powers and whose hollow powers remain domainant. Unlike shinigami whose zanpakuto is a physical manifestation of their spiritual power, a arrancar's zanpakuto is simply their hollow powers sealed into the form of a sword.