Lets say you run a website that allows people to store what they ate for lunch. So you have an input box that asks a simple question, What did you eat today for lunch? Your users enter what they ate for lunch and it post on the website. Simple eh. While your users want to be able to share what they ate for lunch with their friends so you add the ability to do that. Still pretty simple eh. Soon you might add cross tagging and the like to create pretty graphs and statistics to show that one of your users has eaten peanut butter and jelly for the last two years. Still some what simple.
One common thing about the features added above is that they are all back-end features. You don't have to change you design to implement them.