The general term emission-line star refers to any star with emission lines significantly stronger than those of most stars. Among the types of stars that can fit this descriptions:
As per Kirchhoff's laws, the emission lines result from hot material in front of the main source of the star's electromagnetic radiation, the material generally heated by the star's radiation. Among such stars are some pre-main-sequence stars, and some that have ejected material. Planetary nebulae show such emission lines and are discovered in surveys looking for emission line stars, but are generally referred to as nebulae rather than stars, given many are extended sources.