The bastard sword is mostly distinguished from a longsword because it has a grip long enough to allow two-handed use. Bastard swords have slightly longer blades as well, but they remain light enough to be wielded one-handed should the need arise, unlike the greatsword that requires two hands.
A Game of Thrones
A Dance with Dragons
Slung across his back in a black leather shoulder sheath was Longclaw, the hand-and-a-half bastard blade the Old Bear had given him for saving his life. A bastard sword for a bastard, the men joked.—thoughts of Jon Snow