What is electromagnism?

Answer 1

Electromagnetism is a force of nature. It is responsible for all electric and magnetic phenomena such as electricity, magnets, making materials solid, chemical reactions, fire, and almost everything else in daily life except for radioactivity and gravity.

On a deeper level is it associated with a local phase symmetry of nature. This means that one can change the wavefunction of a particle at every point in time or space by a phase factor and the laws of nature would remain the same. Physicists call this the U(1)-gauge symmetry of the Standard Model.