Developing payment gateways

Ubercart supports many different payment gateways either out of the box or by third party modules
These third party modules mostly use Ubercart's payment API. You can find more information about this API by reading the source of one of bundled payment gateways that come with Ubercart, (Look for the payment folder inside the ubercart folder)

There is also another more generic payment API which supports several commerce modules like Ubercart and Drupal Commerce, etc. The payment module introduces this API. So instead of having several modules implementing a gateway like PayPal for different Drupal commerce solutions, it is possible to have only one and prevent duplicate work. Also when payment module gets integrated even with more commerce modules all payment gateways implemented using payment module's API will be supported as well. You can find more information plus API documentation by visiting module's page