Magnetic Field Lines
The charges associated with the electric field are the positive and negative charges on charged particles.
The charges associated with the gravitational field are the masses of particles.
The charges associated with the strong field are the colour charges on quarks and gluons.
For any field with associated charged particles the field lines start at a charge particle and end at another charged particle. For the electric field, field lines go from positive to negative charges.
Because magnetic fields have no associated charged particles, magnetic field lines do not start or end. They must be loops of some sort, sometimes very complicated lops. When the magnetic field line goes from a magnetic north pole to a magnetic south pole, the field actually threads the poles, in a loop, without ever terminating.
Every current or moving charge produces a magnetic field, with direction fixed by the Right Hand Grip Rule. Hold the wire with thumb pointing in the direction of the current, and your fingers curl around the wire in the direction of the magnetic field.