This sword was intrusted to Inu Yasha's father, to Inu Yasha only. Sesshoumaru's demon side makes him incapable of even touching the sword. This sword has many useful techniques, such as the Wind Scar, Backlash Wave, and Red Tetsusaiga- which is capable of breaking barriers put up by demons.
As the series progresses, many attacks become noticeably less useful and some become useless altogether like the Red Tetsusaiga. However, he continues to acquire new weapons as the story goes on. Near the end of the anime, Inuyasha gains the Kongo Souha attack or "Adamant Barrage" in the english dub. Later in the manga he gains the dragon scaled Tetsusaiga that has the ability to drain Youki from whatever it touches. The dragon scaled Tetsusaiga later gains it's true ability to cut the Youketsu of an enemy thus killing it instantly. However, this attack proves to be nothing more than a way for Inuyasha to acquire the ultimate technique of the Tetsusaiga. The Meidou Zangetsuha, which he gains after Sesshoumaru breaks the Tenseiga over the Tetsusaiga. At first, Inuyasha doesn't know how to use the attack effectively at all. However, he eventually creates an attack that uses the Meidou as blades, thus keeping alive the trademark that the Tetsusaiga is a sword that cuts.