They often tell you what is acceptable as well as how to do many things. For example, listserv groups often send out a set of rules when you subscribe. These rules contain important information about how to change your subscription. If you misplace these rules, you may find it difficult to unsubscribe. The unwritten rules are much more difficult.
Online communities can have a long history and be closely knit.
The best thing for a new member to do is to lurk. A lurker is one who watches what goes on without participating in the conversation. Often by lurking, you can learn a good deal about the unwritten rules of the community. While we should respect the privacy of others, we should also realize that not everyone has our values. While most networked computers have many security features, we should realize that network administrators have access and no security system is foolproof.