What are the different version in Windows 2003 and the differences between them

Hi to all!!! I am currently finishing getting my degree as a Network Security Professional/Administrator. I am working with a book "Network + Guide to Networking, 4th Edition by Tamara Banks. I am confused about the Windows Server 2003 and the different Editions. There is Standard Edition, Web Edition, Enterprise Edition, and Data center Edition. I do understand from my reading that the Standard Edition would be for a small working environment. My question is do each of this Editions build upon the previous or would you need to installed each different Edition for let say a large health-care insurance provider. I understand the requirements for each Edition. If anyone has had this class please give me some in sight to this so that I can answer the Case Project correctly. I have had 8 years experience building networks but that was about 5 years ago due to husband's health and death. My class ends Wednesday and I need some advice ASAP, PLEASE!!! Have a great day!