Technical definition of CCA:
Cold cranking amps (CCA) is the amount of current a battery can provide at 0 °F (−18 °C). The rating is defined as the amperage a lead-acid battery at that temperature can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery).
Real World Definition of CCA:
Cold cranking amps are what eventually give out and you end up with a battery that may show 12 volts when you check it across the terminals but doesn't do anything when you hit the starter button.
The more CCA you have, the longer the battery "lasts"...usually.