One of the peeves for mobile developer is that if they are building a server-client app, not only do they need to learn the framework from the client side (such as iOS, Android, WP7), they ALSO need to learn how to code the server side.
On the server side development, it involves a couple of things. You need to setup the server, use a certain web framework, setup database, and create the APIs.
Parse is a development tools that help to get rid of these cumbersome server side development. It provides iOS and Android SDK that helps to connect to Parse backend and store your objects!
It is free if you have less than 200,000 API requests a month. Anymore, you would be paying $9 to $99.