A physical manifestation of Ichigo's soul in the form of a sword. Ichigo's weapon against the Hollows.
Ichigo's sword is a special kind of sword used by Shinigami called a Zanpakuto. Each Zanpakuto has three different forms. They also have a supernatural embodiment that the wielder communicates.
The Zangetsu embodiment appears as an older man in a long cloak, long hair, unshaven, and sunglasses. Though the name of this embodiment is Zangetsu. Ichigo always calls him by the name "Oldman".
After the destruction of the first form of Zangestu. It has never returned to the Sealed Form. It has remained in it's Shikai Form. Only changing when Ichigo would use Bankai.
Ichigo first got his Zanpakuto after he adopted the powers of Rukia Kuchiki. It was an act of desperation in order to save his family from a Hollow that attacked his home. Originally, it was thought that when he took Rukia's sword to his own heart and was able to absorb her powers. It actually awoken Ichigo's repressed Shinigami powers. The Zanpakuto was the largest Rukia had ever seen. It was as long as he was tall.
When Rukia tried to leave Ichigo, she was found by both Renji Abarai and her older brother, Byakuya Kuchiki. They were there to arrest her. Ichigo tried to save Rukia, but Byakuya easily defeated Ichigo and destroyed his Zanpakuto in the process. All that was left was the handle, hilt, and a a few inches of the blade.
Shikai - Second Stage
After Rukia was captured. Ichigo was trained by Kisuke Urahara. It was during this training that Ichigo learned he had always had Shinigami powers and learned to unlock them. With his powers returned it was up to Ichigo to learn the name of his sword. To learn the name was to be able to unlock the Zanpakuto's true powers by reaching the second form of his Zanpakuto called a Shikai.
In this training, Ichigo met in his mind with the embodiment of his sword. He learned the name of his Zanpakuto is named Zangestu.
In this second form, though large the Zangetsu seems rather unremarkable. It looks like a giant unfinished kitchen knife with no handle. Again, it's nearly as long as Ichigo is tall.
Bankai - Final Stage
Though Ichigo had improved. He still required more training to defeat Byakuya Kuchiki. That came from Yoruichi Shihouin. Normally. the training to reach the level of the third form, Bankai, would take years. By using a special training developed by Kisuke Urahara. It was hoped that Ichigo could reach that stage in a matter of days. Ichigo was surrounded in an underground cave by numerous variations of his Zanpakuto. The goal was to find the true one and defeat the human embodiment of Zangetsu.
Ichigo was able to defeat the Old man Zangetsu and unlock Bankai. He first used it in the final battle with Byakuya Kuchiki. In Bankai, the Zangetsu sword was again rather unremarkable in appearance. It was smaller and looked like a solid black katana with a chained charm off the handle.