Any way to have a program with different configurations?

I am doing a program to keep track of user lists and visits for a
local foodbank. Everything worked fine as long as I have only one
possible configuration. Now I have a second foodbank looking into
using the software. They will need a few things changed. There will be
different fields in the database, changed headings in web pages, maybe
more reports. I would like to keep one source tree to make updates
easier, but I don’t know a way to do this. Please help.

                                        Thanks

                         Bob S. <[email protected]>