Retail licenses are purchased from the store when you buy Windows.
OEM licenses come with the computer.
OEM licenses are tied to the computer you have bought.
Retail licenses can be transferred from system, to system, but the system the license is being transferred from must be decommissioned or Windows must be uninstalled.
For Windows 7, the retail licensing agreement permits the installation and use of one (1) license (product key) on one (1) computer. For each additional installation, you'll need to purchase an additional Windows 7 license.
The Microsoft License for use of Windows has always been limited to allowing installation on only a single machine.
MS Office Home and Student edition (Word, Excel, OneNote, Powerpoint) allows 3 installations for a single household.