The bipolar transistor is present as a driver for the MOSFET. Although to DC, MOSFETS have a very high resistance and so look like open circuits, they actually capacitive. In order to turn on, charge has to be transferred into them, and doing that fast requires current driving.
The BJT (and the overall circuit design) also brings in the following advantage: a small and predictable turn on voltage. You can substitute different BJT's in there, and the behavior will be similar.
One more advantage of the extra transistor is that the extra transistor stage has voltage gain, which helps create a sharper transition from off to on, from the perspective of the input looking in.
To use a small, positive signal to turn on the circuit, an NPN transistor has to be used. But the output of this is inverted, with a high-side load, and so a P-channel MOSFET is used. This has another nice feature, which is that the load is controlled from the positive side, and so remains grounded when the transistor is shut off.
The schematic symbol for the MOSFET looks like a depletion device (since the channel is drawn solid, rather than as three sections). This is probably just a mistake. The circuit looks like a run-of-the mill enhancement mode setup.
The P-channel MOSFET activates when the gate is brought low. It is drawn "upside down". Think of it as analogous to a PNP BJT.
The "flywheel" diode completes the circuit for the inductive load when the transistor/switch opens. An inductor tries to keep the same current flowing in the same direction. Normally, that current flows through the transistor loop. When that is abruptly cut off, it flows through the diode loop, such that its direction through the load is the same, and that means flowing the opposite way through the diode. For this continuation of current to happen, the inductor has to generate "back EMF": a voltage whose direction is opposite to the one that was previously applied to it.