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A couple of definitions (courtesy of weather underground): Extratropical cyclones have cold air at their core, and derive their energy from the release of potential energy when cold and warm air masses interact. These storms always have one or more fronts connected to them, and can occur over land or ocean. An extratropical cyclone can have winds as weak as a tropical depression, or as strong as a hurricane. Examples of extratropical cyclones include blizzards, Nor'easters, and the ordinary low pressure systems that give the continents at mid-latitudes much of their precipitation.
[ed Note: that's not gonna describe Irene until it gets past New England, at least.]

Tropical Cycloses. This happens when thunderstorm activity starts building close to [a] center of circulation, and the strongest winds and rain are no longer in a band far from the center. The core of the storm becomes warm, and the cyclone derives all of its energy from the "latent heat" released when water vapor that has evaporated from warm ocean waters condenses into liquid water. One does not find warm fronts or cold fronts associated with a tropical cyclone.

Note that tropical storms regularly become extratropical storms when they get close enough to the pole to get caught up in a front.

[There's also a 'subtropical cyclone', but let's not confuse the issue any further!] :]