SSO is the Single Sign On engine. There are some huge addition to VCenter since 2.5 days. A Lot of things have changed. It is not just a one item install anymore. There are many pieces to this today. Make sure to read the documentation before you even beging to install. There is a boat load of moving parts to VCenter. If you are updating go to 5.1 update 1A.
Documentation Site is here VMware vSphere 5.1
Please read the hardware requirements and the install guides for both VCenter and ESXi.