Active Directory is what is called a directory service, it stores objects like users and computers.
A domain controller is what the server running Active Directory is called.
You can have multiple domain controllers for many reasons, like redundancy so should one server fail, people can still login in and access things like joined computers using another domain controller while the first server is being fixed.

Just is small example of how Active Directory is to be used:
Forest (examplecorp.local)
- Domain (headoffice.examplecorp.local)
-- Domain Controller A
-- Domain Controller B
-- Computer A, users can login to the computer using their account on using either Domain Controller A or Domain Controller B depending on different factors.
-- Computer B, same
-- Computer C, same
- Another Domain (seattleoffice.examplecorp.local)
-- Same structure for local Seattle office

Only one Domain Controller in a domain can handle can tasks like the Global Catalog and such. This is usually called the Primary Domain Controller (PDC) and the task of things like the Global Catalog can be changed to any Domain Controller should that be needed.
Should you plan on using Active Directory then you should read up on it because it is a very complex piece of software and if you configure it wrong you will could potentially spend hours trying to figure out what went wrong.
A couple good sites on Active Directory that I know are:
Microsoft Active Directory Site
Petri IT Knowledgebase by Daniel Petri - Microsoft MVP

Thanx alot