What are Magnetic Fields?

Like electric and gravitational fields, a magnetic field is a construct which helps you determine the force on a pole due to a magnet generating the field. The force drops off as you move away from the magnet just as for gravitational and electric fields. The arrows of the field indicate the direction that a north pole would be pushed. The density of field lines is proportional to the strength of the magnetic field: the more lines the stronger the field, and the fewer lines, the weaker the field. Sometimes we speak of the density of field lines as the magnetic flux, which is the quantity of field lines passing through normal to a surface area. More flux means a stronger field, and less flux means a weaker field.

There is a weak magnetic field around the Earth which we use for direction every time we use a compass (a small magnet which will line up with the Earth's magnetic field lines).