Magnetic Fields
A magnetic field is a region of space where magnetic poles experience forces. There are actually a few other things that experience forces but we'll worry about them later.
We cannot see a magnetic field but we can represent it on a picture by drawing magnetic field lines

The direction of the field is from N to S. (the direction of the force on a north seeking pole)

Magnetic field strength has the symbol B and is measured in Tesla (T)

Flux
We can imagine something flowing from N to S. We call this imaginary substance flux. The lines on the diagram above are often called flux lines.  

Flux has the symbol theta and is measured in Webers (Wb).

The more flux is flowing through a region of space the stronger the field. Magnetic field strength can also be called flux density.

B and are related by the equation