Information in an active directory can be managed using active directory users and a computer snap-on. As the previous answers state, the active directory is a scrolling list that appears on the screen when no specific start-up software is installed on a computer. (Think of a hacked computer on a television program; it typically shows up with a scrolling list before fizzing out.)
The computer snap-on is just a device that allows two programs or machines to be used simultaneously on the same machine. It turns the items into parts of a whole, more or less.
Please note that I do not work in computer engineering. This is gleaned from information off the web, so things may change between when this answer was written and when you read it.
Microsoft is responsible for creating the Active Directory and it is done in order to word in Windows and the networks for Windows. This is a directory service which means that the names are matched to the addresses in regard to the networks available and used. Usually, the average person can recognize an Active Directory because it scrolls quickly down a black screen on the computer if their computer is not defaulted to start by opening Windows.
A computer snap-in is like the name states. It is something on the computer that snaps onto another object so that the first thing works as part of the entire object it is attached to. The information in Active Directory can be managed using Active Directory Users and Computer snap-in.