An intranet is a private TCP/IP-based network of computers on an organization's secure local area network (LAN). An intranet typically contains confidential and sensitive company documents. Each intranet is custom-designed to meet an organization's needs. As such, only an organization's employees have access to it. The employees access the intranet using the same Web browser they use to access the Internet (for example, Netscape Navigator or Microsoft's Internet Explorer).

An intranet is a computer network that uses Internet Protocol technology to share information, operational systems, or computing services within an organization. This term is used in contrast to extranet (see below), a network between organizations, and instead refers to a network within an organization. The term refers only to the organization's internal website, but may be a more extensive part of the organization's information technology infrastructure. The objective is to organize each individual's desktop with minimal cost to be more productive and competitive.
An intranet may host multiple private websites and constitute an important component and focal point of internal communication and collaboration. Any of the well known Internet protocols may be found in an intranet, such as HTTP (web services), SMTP (e-mail), and FTP (file transfer protocol).
Internet technologies are often deployed to provide modern interfaces to legacy information systems hosting corporate data.
An intranet can be understood as a private analog of the Internet, or as a private extension of the Internet confined to an organization.

An extranet is a special portion of an intranet to which only authorized users may send and receive information or conduct transactions. Think of an extranet as an intranet that allows a company's clients to access it. Extranets can be completely outside a company's network or they can be attached through secure measures.
To read more about intranet and extranets visit Intranets Extranets