What is a clamped inductive load?
Let's imagine your example circuit, except with the inductor replaced with a resistor. This is a really simple circuit: when the transistor is on, the current through it is given by V = IR. When the transistor is off, no current flows.

Now, add the inductor back in. The inductor does not like the current flowing through it to change: however many amps it is (0, 1, or 42), the inductor wants it to stay like that (due to storing energy as a magnetic field: you have to 'charge up' the magnetic field to increase current or 'discharge'it to reduce current). It resists complying with V = IR.

So, in the example circuit, when the transistor is turned on, it will take a while for the current to ramp up to its full value. In most cases, you don't really care. The problem comes when you turn off the transistor: the inductor doesn't want the current flow to stop, and keeps ramming charge through itself at the transistor. The energy stored in the magnetic field in the inductor has got to go somewhere. This causes a HUGE voltage spike between the inductor and the transistor that is likely to exceed the voltage rating of the transistor and damage it.

That is what the clamping diode is there to prevent: it will prevent that node between the inductor and the transistor from rising above the voltage of the power supply and damaging your transistor.