Arrancar are powerful Hollows that were altered to allow them to suppress their true powers into the form of a sword. They retain some semblance of their former appearances such as mask fragments. They can release the power sealed in their weapon, unleashing its full abilities, called Resurrección. Arrancar also regain some humanity so are no longer driven by instinct.
A Vizard are a group of former Shinigami that gained the powers of a Hollow (ability to fire a Cero and increased abilities) through a process known as "Hollowfication" making them much more powerful. They access this power by donning a Hollow mask, but Vizard are basically Shinigami with "extra."