Saving data online for IOS
First you need to learn about restful web services. If you are familiar with server side scripting languages like php, then you can create web service for each operation. Here is a good tutorial for creating rest api.
Lets say you want to save chat data from iOS app to server. Here is what will happen.
Client Side (iOS app): Your app will make a request (hit a url ) e.g abc.com/savechat. With this request, your app will "POST" data along with request. e.g sender name, receiver name, message body and time etc. Then server can send response e.g. a success message back to client.
Server Side: Server will perform data operations like storing data and send a response back to client.
For processing web services (JSON) on app side either use libraries like Alamofire for Swift and AFNetworking for Objective C.