A user-defined function is a function provided by the user of a program or environment, in a context where the usual assumption is that functions are built into the program or environment. UDFs are usually written for the requirement of its creator.
Built-in functions
A function which is already defined in a program or programming framework with a set of statements, which together performs a task and it is called Build-in function. So users need not create this type of function and can use directly in their program or application.
Diffrence:
A defined function is one that you define in your own code. A built-in function is a function that already exists in the language without you having to define it.
