Ad-hoc distribution of iPhone OS Applications permits developers to share their iPhone, iPod Touch and iPad applications with a limited number of people before applications are released in the iTunes store, making it possible to give out advance copies for feedback from others. Installing an application distributed ad-hoc can be a bit of a tricky process.
This process assumes that you have already provided the developer with your device ID (UDID), and that he has used that to create a provisioning profile for your device. You will need this provisioning profile to install the application. If you have not provided your device ID, follow the instructions on How to Obtain the Identifier Number (UDID) for an iPhone, iPod or iPad to learn how.