It often refers to the visible light emission spectrum, although it extends to the whole electromagnetic spectrum, from the low energy radio waves up to high energy gamma rays.
The emission spectrum can be used to determinate the composition of a material, since it is different for each element of the periodic table. For example, it is used to indentify the composition of stars by analysing the received light.
Interestingly, the emission spectrum of an element is the exact opposite of its absorption spectrum, that is, the frequencies emitted by a material when heated are the only frequencies that will be absorbed when it is lighted with a white light.
Some examples of emission spectra :
See also Rydberg formula.