In C, data types are used to specify the type of data that a variable can hold. There are several built-in data types in C, including:

1. char: The char data type is used to store a single character. It is usually 8 bits in size and can hold values from -128 to 127.
2. int: The int data type is used to store integers. It is usually 16 bits in size and can hold values from -32768 to 32767.
3. float: The float data type is used to store single-precision floating-point numbers. It is usually 32 bits in size and can hold values with a decimal point.
4. double: The double data type is used to store double-precision floating-point numbers. It is usually 64 bits in size and can hold larger values with a decimal point.
5. void: The void data type is used to represent the absence of a value. It is often used to specify that a function does not return a value.

In addition to these built-in data types, C also supports user-defined data types, such as structures and enumerations, which allow developers to create their own data types with custom properties and values.

It is important to choose the appropriate data type for a variable based on the type of data it will hold. Using the correct data type can help to ensure that your code is efficient and free of errors.