An operation is a very basic calculation based on an axiom : addition, multiplication, division, modulus, substraction, etc ... They can be used on their own or as parts of functions to accomplish something. Example :

,

,

... An operation can be viewed as a simple function.

A function is a combination of operations arranged in order to achieve a particular goal. Take the factorial as an example :

. This is a combination of

multiplications, arranged in order to obtain the factorial of a number.

A function is not necessarily defined as

or

or any other letter.

is a function.

is a function.

is a function. Etc ...

Do you understand better now ?