Document Management System

Question

Hi,
 
I'm working on a project where I need the application to have some document management capabilities. Some of the features I'm looking at are like:
 
1. User can upload various types of files(majorly document files e.g. document,presentation,spreadsheets,text document, etc.).
 
2. He/She should be able to categorize through some tags or should be given functionality to have in separate folders.
 
3. I've created the file upload facility. Currently I'm storing the user uploaded file in a file system. Where each user's files are kept together. Is there a better way to keep them in separate user wise directories or should I use any DB storage scenario...?? Separation is for security and better manageability.
 
4. User should be able to retrieve a file whenever he needs.
 
I came across some of the ready made solutions which can do such things like (Sharpoint, DotnetNuke, MojoPortal, etc..) But I'm more keen into understanding the core concepts of DMS and build something custom in native C#.
 
If anyone can give me a good head-start, some examples or the classes and resources used to build such solution would be of great help.
 

Thanks & Regards,
Jaydeep