The Buffer Tube holds the buffer spring and buffer. Also as the rifle is cycling it will hold the bolt and carrier group. There are several different buffer tube systems, one is the AR pistol buffer tube, then there is the carbine buffer tube with mil-spec and commercial versions, and then there is the standard buffer tube. Now with all that there are also some other custom buffer tube systems. Most people will be going with a standard if they want a fixed stock, or a carbine length. The carbine length is where you need to be aware of the commercial vs mil-spec, since the commercial version is a little bigger in diameter so a mil-spec stock will not fit on a commercial buffer tube. Here is a picture of a commercial carbine buffer tube:
You can purchase several different options, including a full buffer/stock kit here, or just a buffer tube here, and a different place to get one here.
Here is a picture of the installation on an AR15 lower: