What is the difference between the Internet and an Intranet?

The Internet is the public, global, ownerless network; an intranet is an organization's private internal network built on the same technology but closed to outsiders.

Internet:

• Global, public network
• No single owner
• Uses TCP/IP protocols

Intranet:

• Organization's private network
• Same technology as Internet
• Behind firewalls, not publicly accessible

Memory tip:

• Inter-net = "between" networks (connecting everyone)
• Intra-net = "within" a network (inside an organization)

Go deeper:

• Intranet (Wikipedia) — private internal org network that explicitly contrasts itself with the public Internet (and extranet).
• Internet (Wikipedia) — the global, public, ownerless side of the comparison.