Directly from the docs
If you want to change the font-family to Arial for everything, then you need to do a replace of every use of Tahoma to Arial. Well, in skinning you don't need to do this. You can use the :alias feature. A skin selector that ends in :alias is one that never gets written out to the generated CSS-2 stylesheet. Instead it is included in other skin selectors and is useful in that you can change css properties in one place, not in tens or maybe hundreds of places.