So there are two places where events occur.
1. The UI layer. That is in the GUI that user uses. Basically, they are kind of HTML events like, ribbon button clicked, picklist's selected value changed, a textbox's text changed, etc.
2. On the server side. That is when a record gets created, updated, deleted. And there are some other types of events as well like an opportunity is converted to lead, etc.
For the first one, you would need to use JavaScript and REST calls to your outside services.
For the server side events, you would need to use plugins/workflows that you can code using C#. You can also update the system from here.
Ahmad Pirani Green Beacon Solutions