The whole reason for font stacks is that you cannot guarantee that anyone coming to your webpage will have the font that you want to show, on their computer. They could be using a mac instead of a pc, or unix, or they could be using a cellphone, or OS/2. You cannot know what they come to your webpage with, so flexibility and similar-looking fonts become highly useful.
Here's a font-stack:
font-family: Cambria, "Hoefler Text", Utopia, "Liberation Serif",
"Nimbus Roman No9 L Regular", Times, "Times New Roman", serif;
If the persons web browser has Cambria, then that one will be used. If they don't have Cambria it will try to use Hoefler Text instead, and it will keep on trying each of those fonts until it finds one that the persons web browser uses. That's why the font stack ends with more and more generic fonts, like Times New Roman, and finally serif.
If the persons web browser doesn't even have a serif font (which is rare but possible), then the browser will use whatever it uses by default.