Magnetic field: The space around a magnet in which its magnetic effect can be experienced, is called its magnetic field of the magnet.
Magnetic field lines: The magnetic field line is the path along which an isolated N-pole would tend to move if it were free to do so.
They do not exist in reality. This concept is developed to visualize the strength of magnetic field in a region. The region in which the magnetic field lines are closer to each other, the field is considered to be stronger in that region.