GSM: grams per square meter (of paper/card).

In my opinion, when it comes to business cards, more is more. The higher the gsm, the heavier (denser) the paper is, the more quality it exudes. 

If you've ever been handed a flimsy, easily bendable business card, you probably weren't overwhelmed with a sense of quality, longevity or gravitas about the business it was promoting. Even if the design was amazing, but the paper was thin and easily folded, that business card missed it's true potential.

It all adds to the branding experience of the business - heavier gsm, more quality.  (Better brand.)

When you get mass marketing done (shopping catalogues) you would opt for a lower gsm, the lightest you could get (80gsm) because no-one will be holding on to that piece of paper forever, it's a quick-in-quick-out cheap solution. 

So it depends on what you're printing, but when it comes to business cards, I always try and offer the best: 400gsm (that's about the thickest you can commercially get through a printing press).