What do the *-dev packages in the Linux package repositories actually contain?

My question is very basic, as is my knowledge right now. I'm a long-time Linux user, but have only recently taken a major interest in programming. I see that many popular applications in the Ubuntu repositories have -dev packages (e.g. *pidgin-dev* in addition to *pidgin*). My question is this: What is in these packages? How does downloading *pidgin-dev* help one to develop for Pidgin? Is it as simple as a dependency check for the tools necessary to create the plugins for the given application? I was hoping there would be a substantial amount of documentation in the -dev packages, but if there is, I can't figure out how to access it. I'm thinking of something like a man-page, but useful for editing code. Any and all advice more than welcome!