Newsgroups are a lot like mailing lists, but the mechanics are a bit different. There are thousands of lists in the newsgroup system. When your system administrator sets up your account, he subscribes to a number of lists. You can read and submit messages to any group that your site is subscribed to. Every so often, (usually late in the night) all the newsgroups are exchanged and compiled. If you send a message to a newsgroup, the message will be stored on your system computer until the next exchange occurs, then it will be distributed to every other system in the world which subscribes to that particular newsgroup.
Generally you need a special news reader program to read and write to newsgroups, but most web browsers have this capability, so many people read email through them. Many email programs are also capable of reading newsgroups, so they are commonly used as well.