Circular raised beds with a path to the center (a slice of the circle cut out) are called keyhole gardens. Often the center has a chimney of sorts built with sticks and then lined with feedbags or grasses that allows water placed at the center to flow out into the soil and reach the plants roots.
Because these beds are sitting above the ground it permits the air to circulate around the containers and allows the sun to warm them up quicker. This will allot for earlier planting and quicker germination especially in the colder climate areas.
This type of planting will discourage insects and specific vegetable pathogens that can stay in the soil over winter and planting that same crop in that bed will infect the crop.
When gardening in these beds only means you are growing your flowers and vegetables above the ground. You can create your own by building a wooden structure with drainage holes in the bottom and fill it with your favorite organic soil mixture. Concrete blocks, bricks, or just about any material strong enough to hold the garden bed and raise it above the ground can be used.