Ichigo's Hollow self as seen in Ichigo's inner world
Ichigo Kurosaki grew a split personality of his Hollow self in himself. This Hollow self is his evil alter ego, a commonality of most Hollows. He is also the longest-running villain in Bleach history, randomly appearing in almost every single story arc, including the Bount arc and especially the Vizard arc. His presence can also be felt throughout the Hueco Mundo and Captain Amagai arcs whenever Ichigo dons his inner hollow's mask to fight tough opponents like the Espada.
Appearance
His overall appearance is practically the opposite of Ichigo, especially in his clothings. While Ichigo wears a black standard shinigami uniforms, his Hollow self wears a white shinigami uniform with black cloth belts. His skin and hair are also fully white and his eyes are black (on contrary to humans' white eyes) with yellow iris. His overall white color has led to the creation of his fan-name: "Shirosaki" (a play on his white color, Ichigo's last name being Kurosaki, "kuro" meaning black and "shiro" meaning white).
When the Hollow was introduced for the first time in the anime, he had black teeth rather than white as shown in the manga, obviously to further signify the difference from Ichigo. However, this was later changed to normal white when he appeared again in Ichigo's inner world (see above).
He also has his own version of Zangetsu, Ichigo's zanpakutō: On contrary to Ichigo's black cleaver with silver edge wrapped in white cloths, his Zangetsu is silver with black blades and black wrapping cloths.
Personality
His most distinguishing feature is his maniacal laughter and lunatic grins. He is brutal and sadistic, his only reason for existance being to take over Ichigo's power and his body to use it to kill everything he sees. He taunts Ichigo as being weak and unsuitable for being a counterpart of, though he relishes the idea of taking Ichigo's powers for himself.
Control over Ichigo's powers
The Hollow is capable of taking over Ichigo's body and powers when Ichigo falls unconscious in a fight. When he does this, a white mask starts forming on Ichigo's left side of the face and the eyes blacken. (Note: in episode 82 of the anime, his right eye was normal in color. However, in other countries, this was corrected to the proper black.) He is able to ignore most of the injuries Ichigo has sustained up to that moment, and starts fighting like a berserker (much as he does when battling Ichigo in his inner world) with fearsome strength and speed, much more than Ichigo's already abnormal levels.
ichigo in his full hollow form
In the Vizard arc, Ichigo underwent a full transformation into a Hollow. This full Hollow is shown as a white humanoid lizard with long orange hair and slit eyes seen through the eyeholes on its full mask. It displays powerful abilities such as instant regeneration, growth of additional body appendages, and an exceedingly powerful Cero.
Abilities
The Hollow is seen to be much more powerful than Ichigo, overpowering Ichigo very easily. His physical strength is already greater than Ichigo, while he gains any power that Ichigo does, evidenced by his use of Bankai. He is capable of deflecting Ichigo's Getsuga Tenshō with his unarmed left hand, and his own Getsuga Tenshō is incomparably stronger than Ichigo's. His actual limit is unknown, as he was never in any case seen fighting with all his strength.
Ichigo after having gained control over his Hollow self, donning the mask and using his Hollow powers
After Ichigo subjugated the Hollow, he started using the Hollow powers.