A practical and concise classification of electromagnetic fields in general and special relativity is formulated. We give a simple scheme of determination of reference frame co-moving with electromagnetic field which is applicable to all types of fields with the exception of pure null one in which such a frame cannot be introduced in principle, since only this case corresponds to propagation of field with the velocity of light in a vacuum. The co-moving frame is introduced in the energy sense: the Poynting vector in this frame vanishes. As an important example, the Liénard-Wiechert field is considered, and it is explicitly shown that it propagates with a sub-luminal velocity. The mathematical technique is provided in detail.