A data type is a classification of data that determines what type of operations can be performed on that data and the type of values it can hold.

Programming languages provide several built-in data types, including:

1. Numeric data types: used to store numerical values, such as integers, floating-point numbers, and Decimal values (usually used for money)
2. Character data types: used to store single characters, such as letters, digits, and special characters.
3. String data types: used to store sequences of characters, such as words, sentences, or paragraphs.
4. Boolean data types: used to store Boolean values, which are either true or false.
5. Date/time data types: used to store dates and times, including date-only values, time-only values, and date-time values.
6. Array data types: used to store collections of values of the same type.
7. Structure data types: used to store groups of related data items, which may be of different data types. (not available in MDriven OCL)
8. Pointer data types: used to store memory addresses of other data types. (not available in MDriven OCL)
9. Classes and Objects of those classes.

The data type of a variable or value is important because it determines the range of values to be stored in that variable and the type of operations that can be performed on it. For example, arithmetic operations can only be performed on numeric data types, while string concatenation can only be performed on string data types.

Programmers can also define their own custom data types using classes, allowing them to create complex data structures with their own unique properties and methods.