Source: for complete tutorial, follow the link.
1.0 Introduction
Nessus is a great tool designed to automate the testing and discovery of known security problems. Typically someone, a hacker group, a security company, or a researcher discovers a specific way to violate the security of a software product. The discovery may be accidental or through directed research; the vulnerability, in various levels of detail, is then released to the security community. Nessus is designed to help identify and solve these known problems, before a hacker takes advantage of them. Nessus is a great tool with lots of capabilities. However it is fairly complex and few articles exist to direct the new user through the intricacies of how to install and use it. Thus, this article shall endeavor to cover the basics of Nessus setup and configuration. The features of the current versions of Nessus (Nessus 2.0.8a and NessusWX 1.4.4) will be discussed. Future articles will cover Nessus in more depth.
Nessus is a public domain program released under the GPL. Historically, many in the corporate world have ridiculed such public domain software as being a waste of time, instead choosing "supported" products developed by established companies. Typically these packages cost hundreds or thousands of dollars, and are often purchased using the logic that you get what you pay for. Some people are starting to realize that public domain software, such as Nessus, isn't always inferior and sometimes it is actually superior. Paid technical support for Nessus is even available from Nessus also has a great community of developers anchored by the primary author, Renaud Deraison. When allowed to fairly compete in reviews against other vulnerability scanners, Nessus has equaled or outshined products costing thousands of dollars. [ref: Information Security, Network Computing]

One of the very powerful features of Nessus is its client server technology. Servers can be placed at various strategic points on a network allowing tests to be conducted from various points of view. A central client or multiple distributed clients can control all the servers. The server portion will run on most any flavor of Unix. It even runs on MAC OS X and IBM/AIX, but Linux tends to make the installation simpler. These features provide a great deal of flexibility for the penetration tester. Clients are available for both Windows and Unix. The Nessus server performs the actual testing while the client provides configuration and reporting functionality.