Sources and Handlers
What are events?
VBA is event-driven: VBA code runs in response to events raised by the host application or the host document - understanding events is fundamental to understanding VBA.
APIs often expose objects that raise a number of events in response to various states. For example an Excel.Application object raises an event whenever a new workbook is created, opened, activated, or closed. Or whenever a worksheet gets calculated. Or just before a file is saved. Or immediately after. A button on a form raises a Click event when the user clicks it, the user form itself raises an event just after it's activated, and another just before it's closed.
From an API perspective, events are extension points: the client code can chose to implement code that handles these events, and execute custom code whenever these events are fired: that's how you can execute your custom code automatically every time the selection changes on any worksheet - by handling the event that gets fired when the selection changes on any worksheet.
An object that exposes events is an event source. A method that handles an event is a handler.
VBA document modules (e.g. ThisDocument, ThisWorkbook, Sheet1, etc.) and UserForm modules are class modules that implement special interfaces that expose a number of events. You can browse these interfaces in the left-side dropdown at the top of the code pane:
The right-side dropdown lists the members of the interface selected in the left-side dropdown:
The VBE automatically generates an event handler stub when an item is selected on the right-side list, or navigates there if the handler exists.