Extended service means you are in an area where your carrier does not have any of it's own towers, but they have a signed agreement to provide service using a different carrier's towers to provide you with service. Since there is a signed agreement, you do not pay any additional charges.
For example, say you are with AT&T and you are going to Wichita, Kansas and they do not have any towers there, but AT&T does have a signed agreement with Sprint which allows AT&T phones to use those Sprint towers to make / receive phone calls. Because AT&T and Sprint will likely have bi-lateral agreements (meaning Sprint can use AT&T tower in places where Sprint does not have any towers), there is no charge to you.
Roaming network is when there are no signed agreement between carriers. Using the above example, if you go to Wichita, Kansas and AT&T and Sprint does not have any signed agreement, Sprint will charge AT&T because you are now roaming on their network. AT&T will then pass that charge on to you.