Fuzzy is absolutely right. It's not often that I see my cats pant, but when I have it has been after lots of exertion or from a fever (Marius's reaction to some shots recently). Since cats and dogs do not have sweat glands (aside from those on their feet), they cannot perspire to cool their bodies down like we do. Instead, they take in air harder (pant) to cool themselves. By panting, the warm air in their bodies is exchanged by the cooler outside air.