I'm sure jackie_w will provide a more complete explanation, but some quick examples in the meantime:
Sample code for using fonts already installed on your T1 (you'll find a list of the built-in fonts in the Wiki
Sample code for using your own fonts (create a "fonts" folder in your "reader" folder, then copy your fonts into the "fonts" folder):
Don't forget to update your reader.xml file with the proper font names and filenames:
Note: CSS rules can be tricky. The above is bare-bones and just enough to get people started. It will work for many but not all epubs, depending on how they're formatted.