Base 14 Fonts are specific common Type 1 fonts installed as a part of the Adobe Acrobat installation. The Base 13 fonts are these same fonts, minus Zapf Dingbats, and are found in most PostScript printers.
By default, when creating a Screen Optimized PDF with Distiller or any PDF from PDF Writer, the Base 14 fonts are not embedded in the document. Since these fonts are available in Acrobat Reader it is assumed that they will be available to any viewer and embedding would simply add unnecessarily to the file size. However, Distiller (4.x) settings and PDF Writer for Windows settings can be changed in order to embed the Base 14 fonts.