Solving the rendering equation

L is the radiance from a point on a surface in a given direction _
E is the emitted radiance from a point: E is non-zero only if x’ is emissive
V is the visibility term: 1 when the surfaces are unobstructed along the direction _, 0 otherwise
G is the geometry term, which depends on the geometric relationship between the two surfaces x and x’

Photon-tracing uses sampling and Monte-Carlo integration Radiosity uses finite elements:
project onto a finite set of basis functions (piecewise constant)

Ray tracing computes L [D] S* E
Photon tracing computes L [D | S]* E
Radiosity only computes L [D]* E