Netiquette evolved very early on in the days on electronic communications. It really all started with how to answer the phone (did you know that “Hello” is credited to Thomas Edison. He reckoned it a good way to answer the phone. Alexander Graham Bell, preferred “Ahoy”) and conduct a conversation.
Netiquette is a whole plethora of conventions to cover how we communicate on line and what is considered acceptable usage, covering telnet, email, instant chat, blogs and even second life (IBM issued Second Life conduct rules in 2007). There is even an IETF RFC on the subject.
On the whole, they are all about basic courtesy and politeness, pretty much “do unto others as you would have them do unto you”, and privacy – don’t write something you wouldn’t like to be bandied about on the Internet.
A common one covers the use of emoticons: Don’t overdo it – more than one a paragraph it just starts getting silly.