Re: All right, what's all this, then?!
I'm the AC who mentioned Radius/TACACS farther down in the comments as being helpful. Here's a link with brief explanations of AAA, RADIUS and TACACS+. http://www.pearsonitcertification.com/articles/article.aspx?p=2449614
I have no ties to Pearson and am not able to speak to the efficacy their certification course ware.