I want to connect my DIY basic stamp via RS232 MAX232 to the serial port on my laptop. Now since I'm a complete newbie in electronics, what do I do with Ground Pin 5 on the DB9? I assume ground means I have to connect it to some insulator with 0V? Maybe you can also explain why there has to be a GND pin anyway, what is the purpose?
I don't know the STAMP controllers but in this context 0V = GND. All voltages are relative to some reference point. In this case it is 0V on one device and GND on the other device. They just have different names for the same reference point.
If you start going in to mains electrical then 0V may not be GND as this could be Safety Ground (referred to as GND often ) and 0V could be isolated so be careful.
I'll try to keep this answer simple and to the point: Connect ground pin directly to a connection which represents the ground on the schematic in the article you posted. Right under the schematic, there's a list of items on it. Ground symbol is number six on the list. It says:
The GND - GROUND Symbol-symbol indicates GROUND, which in this case is the same as the MINUS of the powersupply.
Note that all ground symbols should be connected together. The actual connection on the schematic is replaced by the symbol with the understanding that on the actual device, symbols will be connected. This is done in order to decrease the amount of clutter on the schematic.