Electromagnetism is the link between electricity and magnetism, and the effects they have on each other. A changing electric current can make a magnetic field; a changing magnetic field can make an electric current. That two-way relationship is why we have motors, generators, and radio. As a school subject, electromagnetism is the chapter that puts those ideas together. Though actually, in casual talk people sometimes use it as a big vague word for “anything with electricity and magnets,” which is close but fuzzier than the physics meaning. It is a science-class noun, more formal than electromagnet, which names a single object.