Depending on the capacity of your coffee maker, they can hold between 4 and 12 cups of coffee. Some decanters have a specially designed lid that comes in contact with the brewing spout of the machine. The intent is that coffee will only flow out when the pot is in place. So if you want to pour your coffee before all 10 cups have been brewed, you can pull out the decanter and the machine stops dripping coffee while you pour. These are very handy features (often called pause n' pour), but it does mean you have to use the precise pot with your machine. So what's an air pot? Admittedly, you don't see air pots all that frequently in home use but they are very handy in retail or commercial settings such as restaurants or offices. They are large thermal cannisters that are intended to hold coffee for a long time while staying hot. The air pot holds in the heat, so there is no need for a warming plate underneath. Most air pots have a dispensing spout or spigot, so you can serve up cups of coffee while it sits on a table or counter. Air pots are much larger than decanters, and can hold several quarts of coffee at a time. Though you can brew coffee with a regular drip machine, and then just pour it into an air pot, they are usually used with coffee makers designed for air pots.
Bunn carries a number of coffee makers for use with air pots, usually using the faster pourover brewing method (as opposed to the drip method). The machines are taller to accommodate the large air pot and are designed to brew directly through the lid. For large-volume establishments, Bunn even has a twin brewer that will fill two air pots at a time. Most home brewers won't need an airpot machine, but for a restaurant or office, there is more need to choose between decanter and air pots. It's really about volume and how quickly you need to have your coffee ready to go. Air pots are also a good idea if you need to have coffee someplace away from electricity (outdoor events). Fill up an air pot, and you can take hot coffee anywhere.