Jitbit Helpdesk is an ASP.NET MVC application written in C#, the database engine being used is MS SQL Server. So you'll need a Windows Server 2003\2008\2012 and Microsoft SQL Server 2005\2008\2012 to run it (free "Express" editions are totally fine). You will also need a copy of Visual Studio 2010 or later (free editions are fine) to be able to work with the source codes since the help desk "sources" license ship as a VS 2010 solution.
You will get the entire source codes and you will be able to make any changes you like. To install the updates you will have to merge our latest files with your changes yourself.
Most of the business logic of the app relies in the "BusinessLayer" namespace (in the "BusinessLayer" folder). You can find classes for working with tickets (creating new tickets, adding notes, attaching files etc), with users, sending and receiving emails etc. in that folder/namespace.
Some of the logic is kept in the "Models" folder, this is where the MVC "models" are kept. The MVC controllers are in the "Controllers" folder. Nothing new, this is a standard setup for an ASP.NET MVC app.
All helpers are contained in the "Jitbit.Utils" namespace. This is our library with reusable helpers that we share among our different projects. You can find a lot of utilities for working with strings, with Active Directory, database, HTML and BBCode parsing, sending support-emails, filtering support-emails from spam, exception-handling, logging etc. etc. Typically you won't have to change anything in that folder
FYI: we use the "Dapper.NET" micro-ORM, the same ORM-engine that powers StackOverflow.com. It's open-source, really easy to use and incredibly fast. Dapper basically extends the "SqlConnection" object with the "Query" and "Execute" methods, but you can learn more about it here if you wish.
But sometimes we do not use any object-relational mapping at all, instead we just execute queries against the database by hand. This is intentional, to keep the code readable, simple and fast.
Some (very few) web-pages in the Helpdesk app use ASP.NET "Web forms" technology. These pages are inherited from one base class (and its inheritors) called "HDPage". All those classes rely in "PageClasses" folder. The "HDPage" class initializes UI language and regional culture, sets up caching, database connection and has some helper methods to determine the "current user ID", check user's permissions and more.
We are really trying to keep the sources simple, readable and well commented to make your programming life easier.
We get a lot of questions like that. Yes, you can modify the source codes in any way you want. You're just not allowed to resell/redistribute your derivative works or the original helpdesk app.
Whenever we release a new version of the helpdesk software, you might want to migrate the changes you have made to source codes - into the newly released version. We recommend simply using a diff/merge tool for that, something similar to Winmerge for example.