Ground in DC circuits.

Hi!

I have some theoretical knowledge of electronic circuits, but when it
comes to practical, real circuit, there are some issues. The biggest
problem is grounding. I read several articles about loops, but it is
not enough.

E.g.
http://chaokhun.kmitl.ac.th/~kswichit/dcsupply/circuit.gif

0 V and GND are at the same node. (?) Does it mean that I have to
connect it to something (to what?) or does this act as ground.

Thanks In Advance.

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