Put very crudely, an XBAP program is half-way between an installed Windows program and a web application. In terms of the complexity and power of the user interface an XBAP program is more like an installed program and can utilise the full power of WPF.

However, an XBAP program runs within the computer’s web browser, unlike an installed program. In addition, unless there is an appropriate certificate installed on the user’s PC, the XBAP program runs in a ‘sandbox’ that strictly limits its ability to directly access the user’s PC. In this mode the XBAP program can really only talk to the web server. This is to ensure that malicious software cannot be spread via the XBAP mechanism.

However, if an appropriate code-signing certificate is pre-installed on the user’s PC then the XBAP program can run completely out of the sandbox and runs just like an installed program, giving it full access to the user’s PC. The code signing certificate ensures that the XBAP program has originated from a particular company and has not been modified, so is safe to run (providing the company is trustworthy of course!).

A good example of the powerful user interfaces that XBAP programs can provide can be seen at:

http://scorbs.com/workapps/woodgrove/FinanceApplication.xbap

A good overview of XBAP can be seen at: http://www.xbap.org/

(Note that XBAP requires .NET3.0 or later to be installed on your computer).