An electromagnetic field is a property of space through which an electric charge is in motion. A motionless electric charge will produce an electric field in its surrounding area, while a charge in motion will also produce a complementary magnetic field.
An electromagnetic field can be see as the product of the interaction between electric and magnetic fields, but as having an existence in space independent of is related charges or currents.
Fluctuations of an electromagnetic field over time are called electromagnetic radiation which manifests itself over a large range of frequencies, from Extremely Low Frequencies (ELF), produced by, say household current, through radio waves, up to visible light and beyond into such radiation as X and Gamma Rays.
Humans spend their entire lives surrounded by electromagnetic fields, whether static - the Earth's magnetic field, for example - or dynamic (electromagnetic radiation) - the light from the Sun.
As the two examples above suggest the majority of background EMFs we encounter are liable to be of a natural source. Any strong - energetic - fields that we are subjected to are likely to be of an anthropogenic origin, produced, in some way, through human agency. This can be anything from a kettle, a computer monitor or a microwave over up to lasers and X-rays.