Magnetic fields are defined in terms of the rotation of a small magnetic dipole.
This makes them quite different to electric and gravitational fields. Fortunately there is a lot you can memorise here, and the calculations required are less challenging as the magnetic field strength (flux density) tends to be constant.
Key Concepts
Magnetic poles
If a magnet is suspended from a string, one end always points towards the earth's geographic north pole. We refer to this section of the magnet as north-seeking.
Earth's magnetic field
Magnetic field lines are in the direction that a north-seeking pole would point.
Field around a bar magnet
Opposite poles attract, so a north pole attracts to a south pole. The field lines for a bar magnet therefore point from N to S.