Create A Network
As of WordPress 3.0, you have the ability to create a network of sites by using the multisite feature. This article contains instructions for creating a multisite network.
A multisite network is a collection of sites that all share the same WordPress installation. They can also share plugins and themes. The individual sites in the network are virtual sites in the sense that they do not have their own directories on your server, although they do have separate directories for media uploads within the shared installation, and they do have separate tables in the database.
Understanding WordPress Multisite, a conceptual
Migrating Multiple Blogs into WordPress 3.0 Multisite
This tutorial explains how to migrate multiple WordPress installations into one new WordPress multisite. You can migrate sites that are using their own domain, as well as sites using a subdomain on your main domain.
Move an Existing Blog into WordPress Multi-Site
Comprehensive WordPress Multisite Migrations
Setup A WordPress Multisite