Norton's theorem states, that the equivalent circuit is a constant current source in parallel with an internal resistance.
The values of the constant current source and internal resistance are calculated as follows.
The formal statement of Norton's theorem is: "The linear network behind a pair of output terminals, can be replaced with a constant current source in parallel with an internal resistance." (The term linear network just refers to a circuit containing components like resistors, to which Ohm's law applies)
Norton's theorem is best illustrated with the example on the following page. This example only uses a very simple circuit to illustrate how to apply the theorem. In practice, the circuits to which we apply Norton's theorem can be much more complicated.