Are you writing a chess engine that you want to have work with XBoard and/or WinBoard? Are you writing a chess user interface or other tool and want it to work with the same engines that work with XBoard and WinBoard? Here are some pointers.
First, read the Chess Engine Communication Protocol document, which describes the protocol that XBoard and WinBoard use to communicate with engines. This document is also included with the XBoard/WinBoard source code, available through the GNU XBoard project page.
Second, go to the WinBoard Forum and ask the folks there for help and advice if you need it.
There are literally hundreds of chess engines (programs that play chess) that work with XBoard and/or WinBoard. I gave up on keeping an updated list years ago, and the old list that used to be here contained so many dead links that I have deleted it.
Some good places to find out about chess engines that you can use include Leo Dijksman's web site and the WinBoard Forum. Also see the other links on my Chess Web Sites page.