What is an Intranet and how have developed in complexity?

An intranet is a secure private computer network that uses internet protocols and network connectivity to securely share part of an organization's information or operations with its employees. Building an effective, productive intranet requires planning, collaboration and technical work to make it user-friendly for non-technical staff, and to effectively manage the large amounts of information that exist in an enterprise[1].
Sometimes the term refers only to the most visible service, the internal website. The same concepts and technologies of the Internet such as clients and servers running on the Internet Protocol (IP) suite are used to build an intranet. HTTP and other internet protocols are commonly used as well, such as FTP. There is often an attempt to use Internet technologies to provide new interfaces with corporate "legacy" data and information systems since the purpose of intranet is to centralize all the day-to-day operations of the staff in one easy-to-use tool.
Briefly, an intranet can be understood as "a private version of an Internet," or as a version of the Internet confined to an organization. The term first appeared in print on April 19, 1995, in Digital News & Review in an article authored by technical editor Stephen Lawton[2]. Since 1995, intranets are evolving and becoming a “must have” tool for every professional organization.

Through years, intranets developed to a more complex “internal websites” which provided to businesses many advantages due to the centralization of “employee driven” procedures. This development of complexity, create the different generations of intranets as follows:
First generation: These are the simplest form of intranets. They are designed only for viewing purposes. Basically, this generation of intranet is very similar to static web sites that the data can be changed only by a programmer. The user cannot enter any information and the content stays the same until someone edits the file on the server.[3] These kinds of “web pages” are called static due to the fact that the information on each page is fixed and does not change in any way.
Second generation: This generation of intranets contains also static pages but allow to the user to carry limited activities such as a “live” form that the user can print it out, write some information and post it to the HR department. This kind of intranets includes data that the user can view only and he/she is not able to add or delete information.
Third generation: This kind of intranets is using database as the backbone information storage. It is designed in order to extract information from and post information to a database. Using this methodology, it becomes more easily to change the content of web pages and users can enter information directly into a central database that can be accessed by other people.
Fourth generation: Each user is handled differently automatically by the system. Personalization comes into play since each user is identified with his/her username and password. The home page can be customized to the needs of each user in order to show whatever is important for the specific user. This generation of intranet is the most profitable for an organization due to the fact that the management is displaying only the appropriate material for every user.
[1] Isn’t it time to use your intranet for more than just email? – Virtual advisor Inc.
[2] http://www.wikipedia.com
[3] E-HR: Using intranets to improve the effectiveness of your people – Bryan Hopkins and James Markham