GitHub Apps are tools that extend GitHub's functionality. GitHub Apps can do things on GitHub like open issues, comment on pull requests, and manage projects. They can also do things outside of GitHub based on events that happen on GitHub. For example, a GitHub App can post on Slack when an issue is opened on GitHub.
For more information about using GitHub Apps, see "About using GitHub Apps."
For more information about building GitHub Apps, see "About creating GitHub Apps."
GitHub also supports OAuth apps. In general, GitHub Apps are preferred over OAuth apps. GitHub Apps use fine-grained permissions, give the user more control over which repositories the app can access, and use short-lived tokens. These properties can harden the security of the app by limiting the damage that could be done if the app's credentials were leaked. For more information, see "Differences between GitHub Apps and OAuth apps."