An Outreach app can contain one or more Outreach client extensions that let you integrate a custom widget directly into the Outreach client application. It also allows you to tap into built-in user workflows such as events or mailings.
With Outreach REST API, you can make programmatic queries to your Outreach instance to collect and change data. Rest APIs offer comprehensive control over most of the objects used in Outreach.Outreach Developer Portal and configure the desired features and extensions. Please note that currently to make use of the developer tools, you need admin access to an Outreach organization. You can also request an Outreach sandbox instance to have a dedicated develop & test environment