Magnetic flux, usually denoted by Greek letter Φ, is a measure of quantity of magnetism, taking account of the strength and the extent of a magnetic field. The flux through an element of area perpendicular to the direction of magnetic field is given by the product of the magnetic field density and the area element. More generally, magnetic flux is defined by a scalar product of the magnetic field density and the area element vector. The Maxwell's equations in the absence of magnetic monopoles requires that the magnetic flux through a closed surface is zero.
The SI unit of magnetic flux is the weber.
The direction or vector of the magnetic flux is by definition from the south to the north pole of a magnet (within the magnet). Outside of the magnet, the field lines will go from north to south.
A change of magnetic flux in a spool of electrical conductive wire will cause an electric current in the spool. This is the basis of the production of electricity. When turned around, that is, running a current through a spool, a magnetic flux will be produced in the spool. This is electromagnetism.
Related concepts: gauss, henry, tesla, maxwell, oersted, weber (Wb), volt, and B-Field.