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...