GitHub Concepts
Internal
Webhooks
A webhook is a mechanism that triggers a HTTP POST invocation into the webhook's external URL, every time a specific event occurs.
A webhook can be installed on an organization or on a specific repository. Once installed, it will be triggered each time one or more subscribed events happen in that organization/repository.
Can be set via UI: Repository -> Settings -> Options: Webhooks.
Webhook Secret
Used by OpenShift for its S2I build strategy.
Pull-Request
Usually, a pull request is initiated from a feature branch and indicates the intent to merge with the develop branch. It is possible to initiate pull requests from a previously cloned repository as well.
The pull requests have to be reviewed, approved and explicitly applied by the owner of the parent repository.
Also see Git cloning.