Volume licenses are for corporations who have a number of PCs to install Windows on, these licenses have to connect to an activation server within the company every 30 days to remain activated.
A retail license is for personal/home use & is for one PC at a time, this can be uninstalled from the PC & reactivated on another PC you own - say you purchase a new PC without an OS, you can install Windows on this & activate with the retails license you purchased.
I hope this answers your question.