Docker Hub has Webhooks for example - once you've pushed a new docker image, it will do an HTTP POST to your Web Hook which might be a Jenkins build or whatever you want it to be. Based on the hook, you can now deploy that new Docker image.
SVN has Pre-Commit and Post-Commit Hooks, once you've committed code, it will do something, this something was often a WGET to a URL which is similar to a Web Hook which was often times used to trigger a Hudson build after each commit.
Gitlab has Webhooks which can be used to Trigger issue updates in Jira.
So in short, web hooks can be explained like this,
Action Happened or is About To Happen -> POST / GET to Web-Hook-Url -> Another Action Happens