What purpose does a python backend serve?
It looks like Bespin uses Python in the same way it would use PHP, if the autors chose PHP and not Python.
If you are a PHP developer, you already are a "back-end" programmer and you already know what it does, the only difference is the programming language that was used to do that.
Some web sites, mostly the huge ones like Facebook or Twitter, consist of more layers than the usual MVC ones. If you look at Facebook, you can see PHP scripts that generate HTML and AJAX responses as the "front-end" and high-performance databases, storage, computation cluster, application servers etc. as the "back-end" (where PHP is rarely used). So what is considered "front-end" and what "back-end" may also depend on how you look at it.
I am primarily a PHP developer, and I have been browsing the source code of a few open-source applications recently(Mozilla Bespin in particular), to find that some of them use a Python "back-end."  I was just wondering what the purpose of this back-end is.  I am assuming it is the same thing as the model in an MVC framework and is used to interface with the database, but I'm unsure.  If I'm right and the back-end is used to simply interface with the database, is the sqlite/mysql server included in the backend, because I didn't see any database config information in the install directions?