In Python, a variable can be associated to objects of different types during the execution of the program. While this feature is desirable as it makes the language flexible and dynamic, it also adds a significant overhead to the interpreter as it needs to look up type and methods of the variables at runtime, making it difficult to perform various optimizations. Cython extends the Python language with explicit type declarations so that it can generate efficient C extensions through compilation.

The main way to declare data types in Cython is through cdef statements. The cdef keyword can be used in multiple contexts, such as variables, functions, and extension types (statically-typed classes).