From TeX on Mac OS X wiki

By default, most typesetting will use the Computer Modern fonts. The bitmap version of these fonts was specifically designed for use with TeX and typesetting originally required fonts to be in this format. Unfortunately, such fonts can look ugly on-screen if the viewer does not support them properly (although they will still look fine printed), so another font format is often a better choice. Today it is possible to use several other font formats with TeX and most current installations will therefore use one of the available postscript versions of Computer Modern by default.

If you wish to use a different font, you must ensure that it is installed correctly and specify it in your document. How to specify the font depends on both the engine and format you are using.