A raised bed garden is built on top of the ground on plain dirt. Raised bed gardens are usually contained by a wood or stone structure that keeps the bed in place. You can plant herbs, vegetables, flowers, shrubs, pretty much anything, in a raised bed. The main advantage is having that dedicated area of improved soil. But there are added benefits, such as better warmth (so you can work it sooner in spring); better drainage; looser, more workable soil; low-maintenance.
Most contained raised beds are constructed from wood because it is easy to work with and it is inexpensive. Note that while using treated wood will certainly retard rotting, you will want to weigh the advantages of longer life against the hazards of toxic chemicals in treated lumber.
Here are instructions for building a simple raised bed frame using untreated white oak lumber, which is naturally more resistant to rot than most readily available woods. Depending upon where you live, cedar may also be an option.