What is the difference between Western and Japanese kitchen knives?

The difference is the blade.  Western kitchen knives and Western-style Japanese kitchen knives are symmetrically double-bevelled, meaning the angle of the slope is the same on both sides of the blade. 

Japanese traditional knives are asymmetrically single-bevelled; meaning one side of the blade is either flat or slightly concave. 

The angle of the blade on a Western knife is usually between 20 and 30 degrees on each side.  The angle of the blade on a Japanese knife is usually between 10 and 20 degrees. 

A lot of Japanese food is often consumed raw, so its structural integrity must remain as close to its original state.  The sharper the knife the less damage to the ingredient, therefore Japanese knives is often the sharpest you can buy.