Usenet was a great invention. It allowed people to ask and answer questions all over the world. It allowed fast delivery of news on a specific subject to subscribers all over the world. And it allows us to share files and data.
What is a server?
In order to connect to the global Usenet and to read and post messages, or download binaries, you need two things: a specialized program, such as NewsReactor, and a server, which links you to what you want.
Most ISP's (Internet Service Providers) have their own servers. The addresses for these servers are usually news.serviceprovider, so the ISP xyz.com would have his news server at news.xyz.com. Also, for many ISP's, simply news works too.
NewsReactor can now try to find a news server for you on the First Time Wizard. If you do not know the address for your ISP's news server and NewsReactor cannot find it, please contact your Internet Service Provider.
There are also free and paid dedicated Usenet service providers, which usually offer better conditions, such as the access to more groups and better retention time (that is the average time a post stays on the server, before it is deleted). You might want to check some of these out.
What is a group?
It is clear that if what everyone sends to the Usenet was in the same place, it would be very difficult to find anything. So it is divided in groups. Each group is usually connected to a specific topic, and so are the messages and files in it.
Groups are also organized in a hierarchy, which can be understood from their names (a newsgroup's name tells us a lot). For instance, we have the alt.binaries.pictures.autos group. Its name tells us that the group is part of the "alt" tree (the largest in Usenet), that it has binaries (a fancy name for non-text files), that those binaries are pictures, and that those are pictures of cars.
The best way to find a group with something you're interested in is to let NewsReactor download the list of groups from your news server, and then search that list for what you want (learn how to do that here).
What is a post?
So you have the means of getting to the Usenet, you have a group to find messages or files in, but what is this talk about posts? Well, a post is one of those messages or files, simply put.
So let's say you wanted to find a picture of a particular car. You'd go to a group on that subject, say alt.binaries.pictures.autos and you would try to find a post with that picture. NewsReactor makes it easy. You just select the group, scan it (more on that here), and NewsReactor shows you the list of posts on that group, at that time. In order to read a plain message, you just need to select it and click a button on the toolbar (check here). In order to download a file, again, just select it and click another button (find out how). You can even post your own messages.