What is the difference between Windows 8 Pro and Windows 8 Pro Pack

To keep it simple, Windows 8 Pro Pack is a subset of Windows 8 Pro.

Windows 8 Pro= Windows 8 + Windows 8 Pro Pack

Windows 8 Pro Pack is actually an add on package which contains softwares like Windows Media Center. It is a separate package because Windows 8 by default doesn't have Media Center. So after installing Windows 8 you will have to download and install Windows 8 Pro Pack to get media Center.
For a limited time (Jan 31 2013), you can download Windows 8 Pro Pack for free from the microsoft site. It si called Feature pack. Here is the link: Download Windows 8 Pro Pack (Feature Pack)

Note: Initially Microsoft had mentioned that Media center would come built in with Windows 8. But later they changed plans because of changes in codec licensing charges and other business scenario changes.