NO and NC stand for Normally Open and Normally Closed. As in, when the switch is just sitting there and no forces are acting on it. NO means that Com (or Common .. to either state) and the NO pin are in connection when the switch is open, and NC is the opposite.
So, say you're using one as a bump switch: You don't want current running though the switch unless you bump into something (and Close the switch). So, you'd connect a line from your Pos. regulated power rail to the COM pin, then run a lead from the NC Pin to your MCU. This means that the switch is open and reading 0 to your MCU unless you bump something. In that instance the switch closes, causes the IO pin on your MCU to read High (ie digital 1) and you have successfully detected a bump.
And if your microswitch isn't labeled then you can just pull out your Multimeter and do some testing on the leads to see what does what.
Hope this helps.