When you go to a party, you think about etiquette whether you realize it or not. A work party is quite different when compared to a rave or a family party. Each of them have social norms and rules to be followed. For example, you can wear a bikini at the beach but not at the opera.
The internet is exactly the same way. Depending on your audience, and how you're communicating, there are different rules to be followed. This is called online etiquette (or netiquette for short). Online etiquette is the correct or acceptable way of communicating or behaving on the internet.