There are many reasons why light may come from an object or source. Reflection, where some other source is responsible, is a major consideration.

Condensed matter emits blackbody radiation that is characteristic of its temperature.

Fluorescence involves emission of light by an atom or molecule when an electron in a more energetic level quickly returns to a lower level. The lower level usually is not the original level, and the light energy usually is shifted to longer wavelengths.

Phosphorescence is similar to fluorescence. Thermal activation returns the electron to the lower level. Phosphorescent states can exist from 10^-4 seconds to hours, and are much longer than fluorescent states.

Incandescence involves light coming from a heated filament, as in an incandescent bulb.

Chemiluminescence is light produced in a chemical reaction.

Electroluminescence is light emitted by stimulating an appropriately designed polymer (as in an LED [local]).

Triboluminescence [local] is light emitted as the result of crushing crystals.