Custom exceptions in Apex are user-defined exception classes that can have their own member variables, methods, constructors, and interfaces. They must extend the system Exception class and have a class name that ends with "Exception". They cannot extend other classes besides the Exception class. Custom exceptions enable more detailed error messages and custom error handling in catch blocks. They can also be rethrown as inner exceptions to preserve the stack trace information.
References:
* Create Custom Exceptions | Apex Developer Guide
* Exception Class and Built-In Exceptions | Apex Reference Guide
* Handling Exceptions in Apex: Creating and Using Custom Exceptions
* Custom Or User-Defined Exception In Apex | Salesforce Developer Tutorials
* Using custom exception types | Mastering Apex Programming