What is "Ubuntu Server 12.04 LTS for Cloud"?
Is this the right server edition to download if I wish to configure a web-based file server? What is the difference between this and the classic Ubuntu Server?
You basically only want to install these packages if you really want to start your own cloud-based service, i.e. a massively large service where lots of clients participate in a large infrastructure that is managed by your Ubuntu Server. The clients do some work for the infrastructure while they are embedded in a "cloud" that abstracts away their massive number.
A cloud basically then provides a service with all those clients being its back-end.
Ubuntu provides this server packages for either running guests or deploying and managing your own cloud infrastructure.
But: If that all didn't really make sense to you and you just have a server sitting at home which only serves a few files, websites, does basic routing and the usual, just download the normal Ubuntu Server edition. It will be enough for your purposes.