PWM (Pulse Width Modulated) power supplies are a type of switching power supply. Pulse Width Modulation is generally used to help regulate the voltage in a switching power supply. This is necessary when the current demand on the power supply or the charging system's supply voltage is not constant. In a standard switching power supply (non PWM), each of the primary windings of the transformer are driven with a 50% duty cycle (actually a bit less than 50%) square wave regardless of the current draw on the secondary or the supply voltage. In a pulse width modulated power supply the duty cycle may vary from approximately 1% up to 50% (although it's not generally that wide of a range). The diagram below shows what the transistor's drive voltage from the control chip looks like for two complete cycles.

Please note that the indicated duty cycle is for ONE of the TWO halves of the primary winding (the primary winding could also be considered to be a single 'center tapped' primary winding). At full power, there will be only a VERY small period of time in which either one winding or the other winding won't be driven. Most control chips (i.e. TL594, TL598, SG3525...) allow a small 'dead time' where neither of the drive transistors are turned on.

Regulation:
You should remember (from the transformer page) that the output (secondary) voltage may 'sag' (from copper and core losses) when current is drawn from the secondary windings of the transformer. An electronic device, such as an amplifier, may operate properly only when the secondary voltage (rail voltage) is very close to a preset value. As you already know, the current draw of an amplifier may be as little as an amp or two when the amplifier is at idle (little or no power output) or it may be significant when producing very high output power. In a standard switch mode power supply, this could cause the secondary voltage to fluctuate greatly. As you already know, you could increase the ratio (primary to secondary) to increase the secondary voltage. Although this would keep the voltage from sagging below a certain point, it may (under some conditions) cause the secondary voltage to exceed the safe operating voltage of some of the electronic components (transistors, capacitors...). In many electronic circuits the range of voltage must remain within a 3-5% window. In a PWMPS, the transformer IS wound with a ratio higher than is needed. But... as with op-amps, there is a feedback circuit. Using the feedback loop, the control chip reduces the duty cycle as far as is needed to prevent an overvoltage condition. When the current draw increases, the duty cycle is increased to maintain a proper output voltage. This allows it to maintain a proper output voltage with a wide range of current draw situations. It also allows the power supply to produce a constant rail voltage with a relatively wide range of input voltage from the vehicle's charging system.

Regulated Amplifiers vs Unregulated Amplifiers:
Highly regulated amplifiers employ PWM switching power supplies. Unregulated amplifiers don't use Pulse Width Modulation to maintain a constant rail voltage. This does not necessarily make one design inherently better than the other. Both designs have their advantages and disadvantages. Read this page if you want to know more about the 2 different designs.