Each variable in C has an associated data type. It specifies the type of data that the variable can store like integer, character, floating, double, etc. Each data type requires different amounts of memory and has some specific operations which can be performed over it. The data type is a collection of data with values having fixed values, meaning as well as its characteristics.

In C programming, there are several basic data types that can be used to define variables. Here are the commonly used data types in C:

1. int: Used to store integer values. It typically uses 4 bytes of memory.
2. char: Used to store single characters. It uses 1 byte of memory.
3. float: Used to store floating-point numbers with single precision. It typically uses 4 bytes of memory.
4. double: Used to store floating-point numbers with double precision. It usually uses 8 bytes of memory.
5. short: Used to store small integer values. It typically uses 2 bytes of memory.
6. long: Used to store large integer values. It can use 4 or 8 bytes of memory, depending on the system.
7. unsigned: Used with integer types to represent only positive values, extending the range of positive values that can be stored.
8. signed: Used with integer types to represent both positive and negative values. It is the default behavior if signed or unsigned is not specified.
9. void: Used to indicate the absence of a type. It is commonly used as the return type of functions that do not return a value or when working with pointers.

These are the fundamental data types in C. Additionally, there are derived data types such as arrays, structures, and pointers, which are built using these basic data types.