OK, lets make this easy, as some of you are really making hard weather of this.
For Office Home and Student you can install it on three computers, which must all belong to one family living at the same address. Other versions of Office have different restrictions.
You can't have it split over multiple addresses... so if your daughter goes off to university thats tough. She can't use it (legally...)
Also you cannot use it for business.
Its important to note that earlier versions of Office had different restrictions - Office XP Home edition for instance allowed you to install on three machines, but only use one at a a time. However, that disappeared a long long time ago.