Using QGIS 2.12.3, within the Field Calculator "Geometry" section, there are certain functions, such as "area", that also have a corresponding term beginning with a dollar sign, i.e. "$area". I don't understand the difference between the two.
For example, in the case of "area", the Field Calculator's description pane states: "Returns the area of a geometry polygon object", while the "$area" description is "Returns the area of the current feature." I confess that I don't understand the difference, nor how to use them in a formula.