A Be star is a B-type star with prominent emission lines of hydrogen in its spectrum. The designation is combined by the spectral class, B, and the lowercase e denoting emission in the spectral classification system. Line emission from other atomic ions might be present as well, but is typically much weaker.
Other observational characteristics include optical linear polarization and often infrared radiation that is much stronger than in ordinary B-class stars, called infrared excess. As the Be nature is transient, Be stars might exhibit a normal B-type spectrum at times, and hitherto normal B stars may become Be stars.
Source: wikipedia, NASA