I cannot figure this out, I read somewhere that the Cache should be in a user context (not only application wide) but I cannot find out how to use that mode... Because of technological issues with AJAX I want to use the Cache instead of Session for example using:

 Cache["UsersID"] = ...

and obviously it has to be specific for each user session?

HI,Dabbi2000 : 

The Cache object lives inside the current AppDomain (ie. ASP.NET Application Scope) and it stores its entries in a Hashtable. When you store a reference object like a DataSet you are actually storing a live reference of that object, which means if you make a change to that object it’s reflected in all other clients that are trying to read that object or currently hold a reference.

More Details here:

http://west-wind.com/weblog/posts/1214.aspx

I hope the above information will be helpful. If you have any issues or concerns, please let me know. It's my pleasure to be of assistance