Usenet
Usenet is a loosely-coupled network of computers that passed articles from one system to another using a store and forward mechanism. An article posted on one computer would pass it on to the computers that it knew about and they would do the same, etc. Eventually the posted article would be copied onto all computers on the network.
E-mail could also be sent via the Usenet network, but the sender needed to know the path from their own computer to the computer of the person on the receiving end. E-mail addresses could get very complicated. For example, an e-mail address that today would be written like 'davek@davbon.com' was written 'gatech!wa4mei!igikpak!davbon!davek'. Gatech is Georgia Tech and was a well-known hub for Usenet. Someone wishing to send e-mail to 'davek' would need to know how to route an e-mail to GA Tech and could then use the rest of the address in order to get the message to 'davek'.
One of the primary uses of Usenet was to route articles of interest organized by topic (commonly called a newsgroup) - referred to as Usenet news articles. Early versions of Usenet used B-News and then later C-News. In the mid-1990's, INN was developed to take advantage of the way the Internet worked versus the store-and-forward manner of Usenet.