I have an interview for a new job within the company I work for. Apart of the job is to know Active Directory, and I am looking for advise for any website that can basically teach me a good understanding of active directory for free.

Wikipedia (http://en.wikipedia.org/wiki/Active_Directory) has a nice write-up on it, and you should be able to find information using Google.

I hope this isn't an IT job, because if it requires knowledge of Active Directory and you have none, and don't know how to go about finding out about it, I would wonder if you are actually qualified for the job.

I second Philly's point. Active Directory is an integral part of managinf Windows servers. If part of the job is to know Active Directory then you aren't qualified for the job.