Saito application are modules that get installed into the wallet. Applications use on-chain and off-chain messages to communicate and can plug into other applications.
If you are getting started programming on Saito, a good place to start is our tutorial series for new developers. Our Github repository also has a directory full of reference modules you can modify or consult if you are curious how the existing applications work.
The following documentation may be useful for those doing more advanced development. The Module API explains what functions you can include in your module. The Events API explain how to listen and respond when system-wide events occur (like a new block being found). The Services API* explains how peers can inform other peers of value-added services they are running.
/lib/templates/modtemplate.js file. This template file defines a number of default functions that create the default behavior for the module. If you overwrite these functions you can customize the behavior of your module, such as specifying what actions it should when it receives a transaction or off-chain message. This API outlines these basic functions.app.connection channel to be notified when these various events happen - this API explains how to do that and provides a short list of available events.