Installation of Active directory on Windows Server 2003 server?

Hello.
I Want to make small network with 20 system. all system are having Windows Server 2003 respectively. I want to what is active directory ? what it does ?
An how i can install on Windows Server 2003??Is it active directory providing security towards each user??

Active directory allows access to users and administrators and allows the administrator to set security up for each user. Active Directory service manages all users or domain and their relationships that make up network environments. Windows Server 2003 makes Active Directory simpler to manage, easing migration and deployment. First of all you have to check all requirements to set up your Active directroy. All system have NTFS partition with enough free space configured TCP/IP properly.operational DNS server . This are minimal requirement for setup AD. Administrative Tools folder has all the AD management tools installed.Run Active Directory Users and Computers.Open the DNS console. See that you have a zone with the same name as your AD domain.Right-click the zone you created, and then click Properties.Check the NTDS folder for the presence of the required files.If you have the SYSVOL and NETLOGON shares, and their location.All of the above is ok then your AD is properly installed on Windows Server 2003 .

Active Directory is collection of network that can contain users, computers, printers, and other Active Directories.Active Directory service allow administrators to handle and maintain all network resources from a single location.The installation of Active directry itself is Very easy and shouldnt give you too much problems.If you have Installed Windows Server 2003 server on your system then follow the procedure .
* GO to Start, click Run, type dcpromo, and then click OK.
* First page of the Active Directory Installation Wizard, click Next.
* On the Domain Controller Type page, click Domain Controller for a new domain, and then click Next.
* Create New Domain page, click Domain in a new forest, and then click Next.
* New Domain Name page, in the Full DNS name for new domain box, type corp.contoso.com, and then click Next.
* Database and Log Folders page, accept the defaults in the Database folder box and the Log folder box, and then click Next.
*shared System Volume page, accept the default in the Folder location box, and then click Next.
*DNS Registration Diagnostics page, click Install and configure the DNS server on this computer and set this computer to use this DNS server as its preferred DNS Server, and then click Next.
*On the Permissions page, click Permissions compatible only with Windows 2000 or Windows Server 2003 operating systems, and then click Next.
* Directory Services Restore Mode Administrator Password page, enter a password in the Restore Mode Password box, retype the password to confirm it in the Confirm password box, and then click Next.
*Summary page, confirm the information is correct, and then click Next.
*Prompted to restart the computer, click Restart now.
When computer restarts, log on to CONT-CA01 as a member of the Administrators group.