You must always first join a list to become a member. Often this process is handled automatically, and you instantly become a member of the list. Other lists require a human operator to vet your application first. Mailing lists can rapidly cause your mailbox to fill up. With a news group, you decide when you want to read the messages in the group.
News groups are by definition open to anyone, whereas mailing lists can be private and closed.
It automatically sends all messages it receives to the members on the list. It also allows you to subscribe or unsubscribe to lists automatically. It understands a limited but powerful set of instructions. There are thousands of different listservers, each with one or more lists that they maintain. The full, official listserv manual is available in electronic form.