Using raised beds for your vegetable garden is not required but it does offer several distinct advantages over having your vegetable garden on the ground.
The raised bed helps to water to more easily drain. This helps to strengthen the plant roots as well as help prevent disease in your vegetable garden.
Having loose soil in your vegetable garden is very important to the plants, helping both water drainage and root development. Using a raised bed allows you better control over the compaction of the soil in the vegetable garden as well as prevent you from standing on the soil, which is hard to avoid in a typical ground-level vegetable garden.
The soil you use in your vegetable garden is critical to its success. Having a raised bed allows you much better control over the composition of your soil. It's also easier to add mulch and compost to your vegetable garden's soil.
Easier to Weed
Since the beds of your vegetable garden are off the ground it is much easier to weed them. Instead of being on the ground you can sit on a stool or the side of the raised bed while you weed your vegetable garden.
Extend Gardening Season
With a raised bed the soil in it warms up quicker in spring and remains warmer in fall, extending your growing season significantly and bringing your crops to maturity much quicker.
Making a Raised Bed
There are many places that sell raised beds for your vegetable garden. The raised beds are normally no bigger than 4 foot wide by 8 foot long and should be raised at least 8-12 inches off the ground. You can also make your own raised bed out of wood, stone, or concrete. Here's a good reference on how to build a raised bed vegetable garden quickly and cheaply
Raised Bed Vegetable Garden Articles
17 Easy steps to quickly and cheaply build a raised bed garden yourself.