Variable, Keyword and constant

Variables are the container/store house for any data that is inputted. To acquire space for the data to provide, space should be given name. Variables are just symbolic representation of memory location. For example:
 a, x, y, abc, ab1 etc

Rules for writing variables:

• A variable can have alphabets, digits and underscore
• A variable name must start with alphabets or underscore but not with digit
• No whitespace is allowed within in variable name
• A variable name must not be reserved keyword

Keyword
A reserved word in C programming is known as keyword. Keyword cannot be used as variable name.
These are the keyword of C programming

1. auto
2. break
3. case
4. continue
5. register
6. char
7. const
8. do
9. double
10. while
11. default
12. else
13. if
14. goto
15. for
16. long
17. return
18. short
19. signed
20. long
21. register
22. switch
23. typedef
24. union
25. struct
26. void
27. volatile
28. int
29. float
30. extern
31. enum
32. volatile

 Constant
A constant is a value or variable that cannot be changed in the program. For example 10, 20, 'a', 'b' etc