Good question! Yes, they are two different approaches to the same concept. The general concept being “Sacred circle”:
The circle is a sacred space that is, metaphysically speaking, between realms. Technically it’s a sphere, a bubble, which encases the ritual area above and below, not just around the perimeter.
So it is cast to contain all that is part of the ritual or working, you and the altar. Some practitioners use it to feel protected, hence the name circle of protection but it’s typically used for raising energy and then releasing that energy for a magical purpose.
The circle can be physically marked but it is mostly a sphere of energy. Gardner’s circles were traditionally 9 foot diameter but depending on whether you are working alone or with others, it can be bigger or smaller.
Remember that the circle is a working space and the Wiccan altar is a work bench. The altar can be placed in the center of the circle or to the side (for example facing North).