A saw is a tool that's used to cut wood. It can be a hand tool or a power tool, and it usually has either a blade or a disk with a jagged cutting edge.
The verb saw means to cut through wood or another material using a saw or other tool. If you are going to build a bookcase, you have to saw the wood into the proper lengths. It's a good idea to measure it twice so you only have to saw it once. When you're faced with a tough piece of meat, you might have to saw at it just to cut off a bite.