Bitcoin doesn't differentiate between the two. You really, really can send BTC without paying transaction fees, honest. I do it 3 times per 24 hours.
That's right, however the official bitcoin client - at least version 0.3.21 and a RC for 0.3.22 - FORCED me to add a fee on every transaction I've made so far. There is apparently no way in the settings to disable this behaviour.
Of course, these transactions were in the ~1BTC range and the coins I sent were only a few minutes old.
I realize it's possible to patch the client to get rid of the forced fees. I'd prefer if the client only suggested a fee, and not force it.