writing a story is the best analogy I can provide. If you had a text file for example, on Windows computer that you added to and saved (re-saved) as you made continuous entries.
I like the iPhone "notes" app but you don't seem to be able to change the note or add to it successfully so I wondered if there was something you could add to and re-save as you went along. Another example being, if you were building a grocery list for the weekend. It would start with a couple of items and then grow from there. Then once you were at the store, the last version with the entire list would be available. I have to think something like that's available for iPhones - I just haven't found it.