The airfield is where air units take-off and land, and where paratroop units can be made. A single airfield can fuel and supply 8 aircraft units, not counting Paratrooper Infantry. Airfields, like other unit factories, have the same resistance, regardless of which faction constructs one. The visual design of each faction's airfield, like other unit factories, varies from one to the next.
If an enemy attack is going to destroy your airfield, the first thing you should do is scramble your Aerial Units. The airfield may be destroyed, but at least your units will remain unharmed for a while longer. This is especially important if you have undeployed recon aircraft, as they can remain airborne indefinitely, until they are shot down by enemy planes or anti-aircraft fire.