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ATX Power Supplys...

What's the difference between a 20-pin PSU and a 24-pin power supply unit (PSU)?

One has a 20-pin motherboard connector, the other has a 24-pin connector! 

OK, there's a little more to it than that.  The 20-pin connector is the older ATX 1.x standard while the 24-pin connector belongs to the newer ATX 2.x standard. 

Here is the pinout of the 20-pin power connector:

Here is the pinout for the 24-pin connector:

Notice that the only difference are pins 11, 12, 23 and 14 (12V, 3.3V, 5V and a ground respectively).  You can get a cable that converts a 20-pin connector into a 24-pin connector (it just splits off an existing cable of the right type), but you will need to make sure that the PSU has ample power for the job at hand (check your manual).  If your motherboard needs a 12v auxiliary connector (which can be 4-pin or 8-pin) and your current PSU doesn't have one, you'll need a new PS (although if you have a 4-pin aux connector and you need an 8-pin one, you can get a converter for that too).

All that said, I generally find that it's less hassle to just get a new PSU rather than bother with converters because they add a lot of additional clutter inside the case.

Testing PSU on bench    Test av ATX spänningsaggregat på bänk

För att prova en nätdel (20 pinnars) 1. Ta ur nätsladden. 2. Jorda 14 (grön) (Det går med ett ombockat gem mellan 14 och 15) 3. Sätt i nätsladden. PSU connector ATX12V forced power on (red wire)