MSDN is [M]icrooft [D]eveloper [N]etwork...it is a subscription service from MS that allows developers to keep on top of the all the latest releases of MS products. There are various levels of subscriptions, the lowest being Standard at about 1k a year..Professional being about $1500-$2000 a year and Universal being close to $4000 a year..
All of the sub's include the OS's, and the basic office package and so on. As you go higher you get more of the packages, with Universal giving you ALMOST everything MS makes. If you were to buy these products individually they would cost a lot more than the Universal subscription.
As far as the CD-Key is concerned, these cd's in the MSDN are keyed and as such you basically install them without having to go hunting for CD-Key's and product OEM numbers and all that rubbish..Regards