A power supply is often described as galvanically isolated and this manes that there is no direct path for the flow of current from the input stage to the output stage, this is a safety feature which is there to protect the load. It also means that the ground reference on the output can be different from that of the input, this is often refered to a floating ground and allows for power supply units to have their outputs wired in such a way as to give either a positive or negative output voltage.
One common use of this is in the telecoms industry where the common voltage is -48Vdc.
As well as power supplies we alao have DC DC converters which are isolated from input to output, again this can have the advantage of protecting the output from any spikes at the input, as well as allowing for a negative voltage to be drawn from the output.
If you are looking for an isolated power supply or an isolated DC to DC Converter then please get in touch and we can help you select something.