Originally Posted by Combine
I thought it was to make it easier to cut limbs off, usually the arabs had curved blades ( simitars ) and cut off peoples heads and such
Scimitars and other curved cast blades of that nature are made in a curved shape as a point of style.
A Katana's curve is a result from the cooling of the blade. Traditional Katana's (and most Japaniese Fighting Swords) are formed from Folded steel, When this steel cools in its shaped form it actually contracts. This causes the blade to curve. The degree of curve on a sword can be an indication of its level of craftmenship.
This is also the reason why longer blades (like a No-Dachi and a Katana) have more of a curve then shorter blades (like a tanto or Washaki)