By default the 'font-family:' property ( which,btw, can be included in the 'font:' shorthand) is a STACK of front families. If the first font is not on the list, it will look for the next family listed.
font-family:"Times New Roman",Georgia,Serif;
uses Times New Roman, if not fund then it uses Georgia, else it uses whatever font the user has set up as his/her serif font preference. Hope that answers your question.