What is the difference between the Internet, an Intranet, and an Extranet?

Internet = public, global, owned by no one; Intranet = an organization's private internal network; Extranet = controlled outside access for trusted partners.

* The three nest: the public Internet around a partner-facing extranet around the employees-only intranet. *

• Internet: A worldwide, public collection of interconnected LANs and WANs. Not owned by any individual or group; uses TCP/IP protocols.

• Intranet: A private collection of LANs and WANs internal to an organization, accessible only to members or authorized users. Uses the same technology as the Internet but sits behind firewalls.

• Extranet: Provides secure access to an organization's network for external individuals (suppliers, customers, collaborators) who need access to specific data.

Mnemonic: Inter-net = "between" networks (connecting everyone); intra-net = "within" one organization.

Visual: Internet (outermost) > Extranet > Intranet (innermost/company only)

Go deeper:

• Extranet (Wikipedia) — best single page for the distinction: an extranet as controlled partner access extending an intranet, contrasted with the public Internet.
• Intranet (Wikipedia) — defines the private internal network and contrasts it with public networks, completing the three-tier model.