If you are not familiar with how airport shuttle services work, you should know that there are two main types of shuttle services, Direct Transfer and Shared Shuttle. A shared shuttle service is typically offered in larger markets, dictated by demand. This type of service will generally have designated pick up areas within a certain radius of the airport and will work a specific route, picking people up on the way to the airport. Thus, this service is similar to what a city transit service would offer. Door to Door service is occasionally offered with this type of service, but of course your rate will be higher for obvious reasons.
A direct transfer shuttle will generally pick you up at the airport and bring you directly to your destination or vice versa. They typically offer door to door service. With a direct transfer service, it may or may not be a shared service, but typically it is not. A direct transfer service will usually provide you with a higher level of luxury and comfort than would a shared service.
The rates for a shared service will be more in line with what a bus service would cost, whereas the cost to take a direct shuttle will be comparable with what an airport taxi would charge. However, with any type of shuttle service, you will usually get a better rate than a taxi. A taxi charges your by meter, whereas a shuttle service will normally quote you flat rate.