How do people make money from developing apps?

For example blackberry app, android app, iphone app etc

You can sell the apps, or sell ads displayed in the apps.

After you finish making your application for your chosen platform, you have a contract with the company that produces that platform (Such as Apple).

This contract states that your application will be sold under your name, on their application database service, and as such they take a small percentage of the money you earn for applications. (Apple takes 30% for example)

So once you have your contact signed, application completed, and you're on the application database, you're ready to sell your application.

At this point, there are generally 3 ways people make money from applications:
Payed applications
Free applications with Ad's
Or more commonly, free/payed applications with in-game transactions for things like extra in-game money.

But how will you get people to find your application?
Well, that's simple. You get your application advertised in other applications. This is not necessary, however it is quite helpful. Many applications get known simply because they are fun or useful and as such go up in ranking.

Another way is by getting spotlit by Apple (if you are using the apple service)
This gives you far more advertisement than ever. Apple only does this for applications they know will sell a lot however.

So that's how people make money from developing applications for mobile devices.

Hope I could help.