Active Directory

Question

hi friends,

i need to learn about active directory and it features. can any body explain about "Active Directory".

My question is If I login to the "WindowsActiveDirectory" by using a single "Username" and "Password", I never wants to type the "Username" and "Password" again in any other modules in the Active Directory.

for ex:-

Once we login into our PC with single "Username" and "Password", we never login again in our local machine like "local mail server login" and "Sql server Login" etc.. how can i do this in Active Directory???

Need reply... It's urgent....