A "blade" is a generic term for many things that are made of metal, have a sharp edge, and cut. A knife has a blade. A surgeon's scalpel has a blade. A lawnmower (grass cutter) has a blade. A razor or shaver has a blade. A plane (carpenter's tool for smoothing wood) has a blade. Most of these blades are not weapons. A "sword" is a very specific kind of blade. It is a weapon of war--an important weapon before the inventions of guns. It is several feet long, heavy, has two cutting edges, and a sharp point. Incidentally, the "w" isn't pronounced; the word is pronounced as if spelled "sord."

To add to Dan Smith's excellent answer: In the context of medieval and fantasy settings, "blade" is sometimes used as a synonym for "sword." - Sir John's sword was the best blade in the land.