Most people who think of ancient Japanese weaponry first think of the katana, or samurai sword. The samurai warrior also used other weapons, including the tachi (long, curved cavalry sword), yumi (assymetric longbow), naginata (bladed polearm) and yari (spear).
The tachi was the sword used by the samurai back when they were primarily a mounted force. The sword had a deeper curvature than the katana and hung from the back with cords, with the edge facing down. The deeper curve allowed an easier draw. The tachi was used against foot soldiers.
The yumi is a bow made of bamboo and various woods laminated together. It is longer than the English longbow and is meant to be used from horseback. Unlike many other bows, the yumi requires its arrows to be nocked closer to the shorter limb. This asymmetry allows the bow to move around a horse's neck and shoot from either side.
After the Mongol invasion of 1274, samurai realized they need to adopt anti-cavalry weaponry and fight on foot. The naginata is a six-foot shaft with a thick, katana-like blade affixed to the tip. It acts as a polearm and was commonly considered the weapon of a samurai woman. All women of the samurai class were expected to be able to wield the naginata, and one was included as part of a dowry.
Japan eventually adopted the standard "pike and shot" of medieval armies when it focused on a conscripted peasant force led by elite samurai. The peasantry needed a weapon they could learn to use quickly; hence, the yari was developed. Compared to other Japanese weapons, the yari was simple. It was a spear that could sometimes have a cross-shaped or hooked spearhead used primarily for thrusting techniques.