An Arrancar (Spanish for "To tear off") is a Hollow that has removed its mask, thus gaining Shinigami-like powers. Over the course of both timelines, a number of them allied with Ben Tennyson, while others became prominent antagonists.
Arrancar are Hollows that gained Shinigami-like powers by removing their masks. The individuals that Aizen turned into Arrancars using the Hogyoku possess powers far beyond that of previous Arrancars. Especially powerful Arrancars were assigned ten numbers by Sosuke Aizen and came to be known as the Espada.
Arrancar are the result of Hollows removing their masks. However, an average Hollow removing their mask would not be a problem for the Gotei 13: to hold their own against the Gotei 13 the ones removing their masks would need to be Menos or stronger. The generic term for an Arrancar is Shinigami-Hollow Hybrid.