Usenet is a collection of newsgroups where the users can post messages and these posted messages are distributed via Usenet servers. Each server holds these posted messages for a certain period of time. This period of time is also known as retention time. Usenet can be compared with an internet forum, but the main difference is that the messages are not stored in one server and are only available for a certain amount of time.
A user posts a message in a certain channel (e.g. newsgroup) and then this message will be distributed via the different newsgroup servers. The other users can login anytime to one of these servers via special software (newreader) and see all new messages in that channel. They can also reply to the newly posted messages and these new posts will also be distributed via Usenet servers. If the retention time of a certain server is 30 days, then these messages will be available just for 30 days in that server.
Downloading from Usenet
Perhaps you have already heard about making a download from Usenet. Downloading works quite similar to post messages. Only a so-called binary file (or short binary) will be posted. The file posting usually occurs in binary newsgroups, which are called binaries. You can recognize these binary newsgroups from the newsgroup name, for example, alt.binaries.games. In order to read messages, reply to messages and download posted files you need a usenet reader.
It is often complex software which needs to be assembled with the downloaded files (that are often posted in chunks) later on. Due to the structure of usenet and the fact that downloading and merging these files could only be done via difficult software, it was long just a playground for computer geeks. Today however, many good alternatives are available, which can be operated easily with the Usenet software. A good example for that is the UseNeXT software which automatically merges partial files. Also it makes use of dedicated servers, so you can download as much as you like at your full connection speed for just a small monthly fee. Retention time of UseNeXT servers is over 1000 days.