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 Smith <bsm2th@gmail.com>