Windows 8 Pro allows you to upgrade from an older version of Windows (XP, Vista, 7) to Windows 8 Pro.
Windows 8 Pro Pack allows you to upgrade from a lesser version of Windows 8 to Windows 8 Pro.
The Windows 8 Pro Pack is just license to move a Windows 8 installation to Windows 8 Pro with Media Center. It does not contain any physical media.
The Windows 8 Pro Pack enables you to move up to Windows 8 Pro. Your
PC or tablet must be running Windows 8 to use Windows 8 Pro Pack. The Pro
Pack makes your PC or tablet ready for business by adding
professional-grade features by adding:
Encrypt your data with BitLocker: BitLocker drive encryption helps protect corporate data on mobile PCs that include a TPM chip.
BitLocker now encrypts hard drives more quickly, helping to keep your
data safe without interrupting your productivity. BitLocker now
supports encrypted drives, too.
Stay safer with SmartScreen: SmartScreen services help protect you from malicious sites and software on the Internet. It checks the
reputation of any new app or website, helping to keep you safe no
matter what browser you're using. That helps prevent malware and other
viruses from infiltrating your PC, or your corporate network.
Connect to company networks with DirectAccess: Just because you're not in the office doesn't mean you're not working. DirectAccess allows
remote users to more securely and seamlessly access resources inside
of a corporate network without having to launch a separate virtual
private network (VPN) connection.
Windows Media Center: Windows Media Center lets you turn your PC into a home entertainment system that can play and record
high-definition TV (if you have additional tuner hardware), DVDs, and
CDs. It's also a software DVR that can play photo slideshows and
Internet TV content.