Newbie Qs (I read the guides)

More appropriately it might be said that any units (or nation) is not hampered by any temperature variance that it has Resistance too. So if units have Cold Resistance, fighting in a Cold 3 Enviroment doesn't change their battlefield condition. Wheras Fire Resistance doesn't hamper anyone fighting in any sort of Heat Enviroment.

All other affects of temperature are on economy as Nagot said.

[Taqwus]

Heat/cold also has some influence on heat/cold radiation (units that radiate heat do it more powerful in Heat-3 than Cold-3, for instance), Caelian ice armor (avoid Heat-3...), and certain spells (Heat from Hell is most powerful in a Heat-3 dominion, Murdering Winter most lethal in Cold-3 et al).