A user-defined data type refers to a data type that is created by the user or programmer in a programming language. It allows programmers to define their own custom data structures to suit specific needs. By defining a user-defined data type, programmers can encapsulate related data into a single entity and perform operations specific to that data type.Here's an example to illustrate the concept of a user-defined data type in the C programming language:```c// Defining a user-defined data type called "Student"struct Student { int id; char name[20]; int age;};int main() { // Creating variables of the "Student" data type struct Student s1, s2; // Assigning values to the variables s1.id = 1; strcpy(s1.name, "John"); s1.age = 20; s2.id = 2; strcpy(s2.name, "Jane"); s2.age = 19; // Accessing and manipulating the data printf("ID: %d, Name: %s, Age: %d\n", s1.id, s1.name, s1.age); printf("ID: %d, Name: %s, Age: %d\n", s2.id, s2.name, s2.age); return 0;}```In this example, we define a user-defined data type called "Student" using the `struct` keyword. It consists of three members: `id`, `name`, and `age`. We then create variables `s1` and `s2` of the "Student" data type and assign values to their respective members. Finally, we access and print the values of the individual members of each variable.By creating this user-defined data type, we can easily organize and manipulate student-related data as a whole, making the code more readable and maintainable.

Data structures that are already present in the system and are provided by the programming language are called Built in data structure.In case of built in data structure, one can not edit the data type as it is already there in the system.Example- int, float etc.Data structures those are derived from built in data types and are defined or initialised by user are called user defined data structure.Example- in C/C+ can be done by typedef keyword.