For practice purposes, I am looking to build an application that is capable of connecting multiple devices through the internet. It will be similar to craigslist, but I want to make this as an iOS application. I have very little experience with web services, as the most I've done is pulled an RSS feed onto the screen.

How are these things normally done? If it's similar to a message board, is it as simple as having a database in a server/cloud, and giving all users access to it? Or is it more complicated than that? How should I begin to learn more about the backend?

What kind of services are usually used in this kind of thing? The only database I've used is Core Data.