Neko Kenshi is correct. Japanese swords are traditionally made to bend, and not return to their original alignment, on side impact. They are made with a very hard edge, which allows them to stay very sharp. However, the harder steel is, the easier it is to fracture. The Japanese overcame this by differentially hardening the katana. This gives the katana its characteristic curve, and also results in a hamon along the harder edge when it's properly polished. It allows for a very sharp edge at a typical Rockwell hardness in the 60s, but the much softer back, at a typical Rockwell hardness in the 40s, keeps it from breaking. European swords are usually hardened at the spring steel level, with a ROckwell hardness in the mid to low 50s. This allows them to flex quite a bit, but still return to their original alignment without bending permanently. It also means that they cannot hold as sharp of an edge as Japanese swords.