Well, I got this product key from the promotion by Microsoft back in January, and just used it to install MCE on my Windows 8 Pro laptop today. Does it install anything more than this? Is that it?
Windows Media Center is a software which is designed for recording live tv as well as playing music and videos.
It's just like XBMC and used to come preinstalled with high editions of Windows OSes.
MCE does not come bundled with Windows 8 so you need a key to install it, Microsoft gave away some free keys instead of chargin g people for it, you got a free key so you do not need to pay for it that's it.
I mean, do I get something like DVD decoder or anything similar?
No just Media Centre think they where charging about $9.99 for it so you have it free.
What you can do is install Media Center. That is what the key is for!
one license for one product
The entire purpose of the key is to allow you to install Media Center on your system. That's it.