There are basically two different kinds of board bags, travel bags and board socks.
Board socks are basically little fabric covers that you slip over your board to protect it from getting dinged while the board is in the back of your truck (or wherever). They are not meant to, and wont protect your board on an airline flight.
Travel bags are made of Nylon and are stuffed with a small amount of foam padding to protect the board. They have a zipper that runs down the side to get the board in and out. Travel bags by themselves also will not protect your board on an airline flight. However, if you buy a travel bag that is significantly bigger than your board, you can stuff towels, wet suits and whatever else along the rails of the board inside the bag (see tips section). This will protect the board from dings and Murphy’s Law. Travel Bags run about 70 bucks whereas board socks run about 25 bucks. Victoria makes both of these products. I do not really see the point of board socks however, because boards rarely get dinged in your trunk. My advice is to save up and buy the real thing!