A weed is simply any plant that you do not want in your area that you are working with. Weeds are in the yard, flower bed, driveways, sidewalks, and in the gutters if not cleaned out. Any plant that puts off a seed can turn into a weed. There are broadleaf weeds which are seasonal and can be controlled fairly simply by using a 2-4D amine type product. Then there are grassy weeds such as crabgrass, dalis grass, and Bermuda grass in some parts of the country. These are much harder to control, but can be done with specialty products.
The best way to control weeds is to prevent them. Treflan is my favorite way of preventing weeds, but can only be used in the flower bed areas. Use a pre-emergent designed for the yard to prevent in the grass areas. Check out my Ways to help prevent weeds in the flower bed blog from a couple of weeks ago. It goes more into depth on ways to prevent weeds.