What is a switching power supply for amps and

A switched power supply generates high frequency noise - lots of it. That's the major disadvantage. The advantage over a linear supply is that the overall size is much smaller, requires only a 5 volt starting power at the switch (instead of a line-voltage switch), rectifies the power down to low voltages and can provide much smoother dc.

A switched supply is mainly used for computers (the 5 volt push button start switch as opposed to the old 120 volt red switch), digital gear with standby mode and other similar electronics. The reason they generate noise is that they increase the mains frequency via diodes and transistors from 60 Hz to 20 kHz which enables the use of a smaller transformer and filtering capacitors. Because of the RF noise, I wouldn't want one of these in my amp or preamp - but I wouldn't rule it out because of it. As always, it's the sound not the specs.

Chord uses them in their amps.

NAD championed this approach way back in the late 70's and on. They use a power supply with two rails(one low voltage and one of higher voltage)off of a multi-tapped transformer. The first power supply carries continuous capabilities and the higher rail supplies voltage for instant peaks. You can make a smaller amp sound more powerful than its rating would initially indicate. The problem is in the switching. If it is engineered correctly, it works well even though the switch between supplies can be seen on a "O" scope. Since the switch is done at a higher output(usually the maximum continuous output point), the switching distortions are below an audible level. The drawback is the high rail only holds up for a brief period of time(as in miloseconds) Then the amp falls back on the low rail reducing the maximum output substantially. It does make a smallish amp sound fairly strong since a lot of output is only required during brief peaks. This allows the manufacturer to economically build an amp that has high peak capabilities without the added expense of a continuous power delivery.
For an example, the NAD 304 from the early nineties had a continuous rating of about 35 watts into 8 ohms 20-20k. It had a peak wattage output of 120 watts per channel at the same spec. This peak wattage was useful because the amp would hold it for almost 100 msec.
The amp sounded alot more powerful than its rating would suggest. Sterophile actually gave it a "Class C" rating and it retailed for $329. Quite a bargain considering it had excellent sonics and would drive most speakers fairly well.