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 possibility of taking a humanly shape. An 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 guaranteed to become human-like.

An 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 dominant. Unlike Shinigami whose zanpakuto is a physical manifestation of their spiritual power, an Arrancar's zanpakuto is simply their hollow powers sealed into the form of a sword.