This is also called an inverted T foundation as a wider footing is poured below frost line and then a stem wall is formed on top. This can be from concrete block, Insulated concrete forms (ICF) or just cast in place concrete. Once the stem wall is done then the dirt is back filled against the stem wall. This foundation is used when radiant floor heat is placed in the slab as insulation can be put between the dirt and the concrete to reduce in energy lost.
A version of this type of foundation is used when a basement is built just that the spread footing is placed at the bottom of the basement and the stem wall is now the height of the basement. Before the dirt can be back filled the stem wall must be waterproofed so that it does not leak into the basement.