Types of events

The kinds of events that can occur can be divided into two main categories:

User events
System events

Regardless of how the event was called, C++Builder looks to see if you have assigned any code to handle that event. If you have, then that code is executed; otherwise, nothing is done.

User events

User events are actions that are initiated by the user. Examples of user events are OnClick (the user clicked the mouse), OnKeyPress (the user pressed a key on the keyboard), and OnDblClick (the user double-clicked a mouse button). These events are always tied to a user's actions.

System events

System events are events that the operating system fires for you. For example, the OnTimer event (the Timer component issues one of these events whenever a predefined interval has elapsed), the OnCreate event (the component is being created), the OnPaint event (a component or window needs to be redrawn), etc. Usually, system events are not directly initiated by a user action.