Magnetic Field Lines : They are hypothetical lines which tell us about the strength of a magnetic field in a particular region such as near a bar magnet.
They are used to represent magnetic field in a region. They are closed continuous curves.
Tangent drawn at any point gives the direction of magnetic field. They never intersect.
Outside a magnet, magnetic field lines are directed from north to south pole and inside a magnet they are directed from south to north.