Almost every app needs to persist data. You have just downloaded an app, worked with it for a while, and the next time you open it, you would like to see that it remembers what you have done so far. An app can store its data in the cloud, in an embedded database or in a file. This last option is the most easy to use. A local file can store information in different formats. It could be a binary file, which is just an array of bytes, that is left to an app to make sense of, or it could be a text file. Your app can store information in plain text or it can use some of the file formats, such as JSON or XML, to make it easier to process text information.
Imagine that you would like to write a small mobile app to keep track of your favorite locations on the internet. To keep it simple, it could be just a list of favorite items made of two strings: an URL and a caption...