What is the difference between system event and custom event in Salesforce?

Hi,

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System Events. The framework fires several system events during its lifecycle. You can handle these events in your Lightning apps or components, and within theSalesforce mobile app.

Event-driven programming is a programming paradigm that allows developers to write functions to respond to fired events. The framework waits for events to be generated from user interactions such as button clicks and mouse overs, or by non-interactive actions such as loading completions or component renderings. When an event is fired the function(s) that have been declared to handle it are called by the framework.

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