Usenet is a worldwide distributed network of "bulletin boards", where people discuss about every imaginable topic (and many topics you didn't even know existed before).
You participate in the Internet, so you also have access to the news server (the "bulletin board") of your provider. You can find out how to access it in your provider's documentation. If everything is configured correctly, you should get access to the news server by clicking on the following link, namely to the newsgroup de.newusers.infos.
You can find the detailed conditions under the link: how much does usenet cost
This "bulletin board" is located locally on a server of your provider, so he provides disk space for it. The news server is one of the services provided by 
your provider, just like mail or WWW. The "bulletin board" is now organized by topics. Depending on the provider, this can be several thousand, or even several tens of thousands of topics. Each of these topics is represented by a so-called newsgroup. As already mentioned, there are all possible and impossible topics: From dogs as pets to world politics, from religion to knitting, from soccer to sex. Simply everything.
So what do you get out of it? Well, let's stay with the example of "dogs as pets". Let's assume you also have a dog and are therefore interested in this topic. You look at the corresponding newsgroup (for the German-speaking area this would be de.rec.tiere.hunde closer, and find there articles on this topic, which other people have written. It can be that someone reports about an amusing experience with his dog, or someone describes a strange behavior of the dog, or someone asks about the optimal coat care for his dog, or ...
I just wrote "article." This is the basic unit in Usenet: the article. An article is written by a person and posted to the newsgroup. This person - the author - is responsible for the content.
So how does an article get into the newsgroup, and how does it get onto your provider's news server?
Let's play through this with a concrete example: You have a problem with your dog, namely he greets all visitors much too stormily, and you can't and won't break him of this habit. So you post a request in the newsgroup. There are thousands of other dog owners reading, maybe someone knows a solution.
So you formulate your questions and "post" them in the newsgroup. Thus the article is available once locally on the news server of your provider, and can be already read by other customers of the same provider. But this is not enough, after all, all people interested in this topic all over the world should know about your question.
Let's assume that there is someone in Australia who has already had this problem with his dog and has solved it. He could send you the solution by mail, but since it is of general interest, he also writes an article and posts it on the news server of his provider.
A short time later, the article arrives on your news server. Sometime after that (a few hours, a few days), you check again to see if there are any new articles, and find that someone has given you a tip. Now you can try it out - or not. Because in the meantime someone else (this time someone from London) has written an article in which he has described the first tip as completely wrong, and suggests another solution.
Now, for your article alone, it would not be necessary to build such an infrastructure. However, there are several million participants on Usenet, many of whom write articles - others just read and thus passively participate in Usenet. Every day, tens of thousands of articles on various topics are written and distributed worldwide.