Say you are a reference librarian and you are stumped by a certain question. None of your fellow librarians at work that day know the answer either. What if you could ask thousands of reference librarians at the same time? One of them could have answered that same question last week and would be happy to share the answer with you! Or maybe you know something which will help answer a question for one of these other librarians!
This is the promise of Usenet. The drawback is that you have no idea whether the person leaving a given message is in fact an expert or a clever wannabe who wants to fool people into thinking he knows what he is talking about. (A person can pretend to be anyone he or she wants to be in "cyberspace". This anonymity means that celebrities can join in the conversations just like ordinary people or it can mean that a dishonest person can take advantage of innocent people, including committing crimes against them after they have lured them to a face-to-face meeting. Most people are honest but, just the same, you would never give out your home address or your home phone number to people you don't even know!)
So, a number of Usenet message areas are "moderated". The moderator makes sure that people stay polite to one another (reasonably, anyway) and focussed on the topic at hand. (Each such area is devoted to a listed topic, and the ground rules are the same for most such areas.)
Before leaving your own messages, it is best to be versed with Netiquette, the Internet version of general etiquette (yes, we're talking Emily Post here!). People who flout Netiquette are likely to find themselves "flamed". (This means that all of those people you were hoping might help you instead send you nasty email messages and overflow your email mailbox and quite possibly crash your computer due to the suddenly crushing volume!)
Usenet is great if you are the type of person who wants to hear what other people who share your interests have to say and if you want to contribute to the "conversation" (called "message threads") yourself. Anyone can join the conversation but it is recommended that a new person read the FAQ and also the "postings" (previous messages) first for a while to get a good idea about what is currently being discussed.
The FAQ is literally a list of Frequently Asked Questions. This is to help new people by filling in the background of what has already been established so that people who have been in this Usenet area for a while won't constantly be interrupted by basic questions that have been answered countless times before. Sometimes, you might learn what you needed to know by just reading the FAQ and you don't have to ask your question at all!!
There is a Usenet topic area on just about any topic you can imagine, scientific or recreational. There is even a Usenet topic for new Usenet users!