Question

differentiate between user defined function and built in functions

Answer 1

IDE also includes third-party functions and some of which are common to developers and such string.h for string handling functions. These is considered because they are common to developers though they are strictly user-defined functions which must be included in programs to use them.  

Windows.h isn't considered built-in since it's only relevance to Microsoft Windows platform.

Built-in function is function, operator or statement that is given as part of standard language and is common to developers regardless of platform.  

Example if(), while(), for(), return() and switch() are examples of built-in functions.