When disconnecting a battery from a vehicle you make sure you disconnect the Negative first and the Positive Second because if you were to disconnect the Positive First you run the risk of arcing the wrench/ratchet to a grounded point (i.e.: Fender, Valve cover etc.) and causing the battery to explode and do some serious damage to your electrical system and most importantly yourself. When re-connecting a battery make sure to connect the Positive First and the Negative Second for the exact same reason.
Remember:
Disconnecting = Negative 1st Positive 2nd
Re-connecting = Positive 1st Negative 2nd