Q:

Differences between FEN and EPD?

I just read the description of FEN (Forsyth–Edwards Notation) and EPD (Extended Position Description), I have a basic idea of what they are, but I still have some points that are not entirely clear:

What are the main differences between those two?
When should I use one over the other one?
Which one is more common to see?

I am trying to decide which one to use for a personal project, and I can't take this jump without knowing everything about them.

A:

What are the main differences between the two?

Extended Position Description is a standard for describing chess
  positions along with an extended set of structured attribute values
  using the ASCII (American Standard Code for Information Interchange)
  character set. It is intended for data and command interchange among
  chess playing programs. It is also intended for the representation of
  portable opening library repositories. The first four fields of the
  EPD specification are the same as the first four fields of the closely
  related FEN specification. Like FEN, EPD can also be used for general
  position description. However, unlike FEN, EPD is designed to be
  expandable by the addition of new operations that provide new
  functionality as needs arise. More information can be found at this site.

When should I use one over the other one?

EPD is more expandable by the additon of new operations and is more
  versatile and is used primarily in chess programs.  See bullet point one.

Which one is more common to see?

Overall, I would say FEN is more common, but this is relative to what you are trying to do.

