I am going to be taking over from a developer here at work soon. Hence, I'll be supporting all the applications that he has written over the last few years.

My question is, when supporting an application that you probably don't know much about, what kind of documentation is most helpful to get a handle on how to fix problems, extend functionality, modify functionality, etc? I'm thinking it would need to give you an overview of what the software does, what interfaces it has to other software, what databases it uses, usernames, passwords, and so on.

Is there such a thing as a software support document? Referrals to any templates would be most helpful.

BTW, unfortunately, there are no requirements documents, specs, etc! So, really my question is, if my colleague had a day to write a single document for each application so that I could (more easily) support it, what would that document be and/or what would it look like?