Katanas are the very famous traditional cut-emphasis sword of "Japan". Invented in the 11th century AD (as the tachi), they all possess the same basic profile; a two-hand grip, a small circular guard, a moderately curved blade, coland (with a few rare exceptions) a long single edge.
Katanas have a blade length no less than 24 inches but no longer than 32 inches; shorter swords were legally considered (in the Japanese feudal period) "Wakizashi" and lawful for non-samurai to bear. Longer bladed swords fall into the dai-katana category (32 inch - 36 inch blade) or the specialist no-dachi (36 inch - 42 inch blade) which is the Japanese equivalent version of the European Great Sword. A shorter (24 inch blade), more greatly-curved katana known as a "chisa-katana" was usually reserved for close-quarters indoor combat within one’s home.
While swords were third-priority weapons in Japanese warfare, (bows and eventually muzzle-loading firearms were first priority and polearms such as spears ("Yari") and halberds (naginata) were a second priority) a highly reverential “cult of the sword” developed in Japan through the Warring States and Tokugawa periods. The most sophisticated Japanese sword techniques were actually developed during the enforced peace of the Tokugawa shogunate (1601-1873) when unarmored duels and other informal encounters became the norm of samurai conflict.
A well-made katana can be a highly effective weapon for battling the walking dead. Being slightly thicker than its European counterparts, a katana is thereby slightly heavier than other swords of the same length, but this is not very noticeable with the normal two-hand method of use. The katana can easily cut through unarmored targets as long as the proper draw-cutting technique is used. This draw-cutting method is not innate (like linear hitting or chopping is) and requires considerable training to perform consistently, effectively, and safely. A skilled katana user can easily behead a human (breathing or otherwise) with a two-handed draw cut, however, an unskilled user can easily botch the cut—as Yukio Mishima’s lover-acolyte did during the famous author’s public "Seppuku". A “mall ninja” who just grabs a katana and starts hacking away at zombies will become zombie-chow in very short order.
During the Japanese feudal period, swords were often tested by cutting several bodies (of executed criminals) in half, the most common being two-body-blades, but going up to six-body-blades. What must be remembered is that test-cutting of human bodies was performed by highly skilled professionals, using extra-long grips for increased torque, and under ideal (non-combat) conditions.
Generally, curved swords are more effective cutting tools (Japanese or not) than straight swords against unarmored targets. The katana’s hilt and blade curve reduce the effort needed to produce devastating draw-cuts when combined with the proper hip and shoulder twisting actions (torque) found in Japanese swordsmanship.
The traditional Katana blade is composed of two pieces of steel, each having a different carbon concentration. This allows for the back which has a slightly lesser concentration of carbon to be flexible while the edge is very hard. Both are assembled before tempering the blade (the action that gives it its solidity and curve) as the centerpiece is "sandwiched" in the edge piece that was welded in a U form. To temper the blade, the swordsmith places it in a fire and then quenches it into water. The center part retracts more due to its different composition, giving the blade its famous curve.
While the entire Katana edge is sharp, it is sharpest usually anywhere from 3-12 inches from the tip. Some well-made katana tips ("Kissaki") are even comparable to a modern scalpel. This was done to encourage the samurai to keep their distance when using the weapon; this same advice will be helpful in the fight against the undead. Properly forged Katanas have a “distal taper” being slightly thicker nearest the guard than the tip; this aids in making the Katana stronger nearest the hilt for making parries and aids in balance as well. Traditional katanas use a hybrid-convex blade and edge profile which, when combined with the very hard tempered edge and careful polishing-sharpening, creates a very long lasting, low-drag, draw cutting blade.
The katana is not without drawbacks. A katana has only one sharp edge as opposed to a double-edged European sword; this was primarily meant to allow the katana to block with the dull back to reduce the risk of damage to the blade. The polishing-sharpening method also created a very “porous” blade surface that is highly vulnerable to rusting, notches, and chips. The butt-end of a katana cannot, unlike many other swords, be used to strike a hard surface like a human skull - such “pummeling” is very likely to destroy the wood grip in very short order. The 3/4 length katana tang is held to the wood grip by a single bamboo pin—inspections of this potential weak point must be frequent, especially if the katana is being used regularly in combat or practice. Also, the cloth-cord grip wrapping needs to be periodically tightened or even replaced when ruined by sweat or blood. Finding a combat-ready katana will be rare, and any average joe with no formal training blindly swinging one will ensure a quick death.
The great majority of “katanas” found in the West are not actual weapons, they are mass produced stainless-steel “wall-hangers” that can come apart with a hard swing in the air, let alone contacting a substantive target like a human body. There actually are some very good introductory “use” katanas being manufactured in China—they retail for about $800 and it would take a high-level "Kenjutsu" expert to discern or obtain any actual combat difference between these affordable blades and any heirloom custom made katana.
Overall the katana is one of the best melee weapons against the undead as long as a user is able to learn the basic draw cutting technique. In terms of cutting, many, professionally opinioned or not, consider the katana a great weapon. Even though a katana can last a very long time, it does require cleaning after every fight as soon as possible to avoid damage.
As he has stated at public speakings, the katana is the favored weapon of Max Brooks, writer of The Zombie Survival Guide. Brooks claims that until the lightsaber from Star Wars is made real, the katana is the best melee weapon to use against the undead. Many actual sword experts and aficionados disagree with this gushing assessment.