We can control the flow of a program by calling a function. If we call a function, it executes, returns a value, and then resumes the program where it was called.
In programming, a named section of a program that performs a specific task is called a function. In this sense, a function is a type of procedure or routine. Some programming languages make a distinction between a function, which returns a value, and a procedure, which performs some operation but does not return a value.
Most programming languages come with a prewritten set of functions that are kept in a library. You can also write your own functions to perform specialized tasks. 
We use function so we don't need to repeat ourselves. Please watch the video below and remember the content.
The difference between returning and printing
Students often print from within a function. Please understand the difference between printing a result from a function and returning a result from a function.
return: gives the value to the program. Callers of the function then have the actual data and data type (bool, int, etc...) return 3 would have the value 3 put in place of where the function was called.
In general, you should
return a value from a function and not directly print from a function.
Example of a function
1 # 2 # this is a simple function that remembers who like hamburgers and who doesn't like hamburgers. 3 # 4 5 def likesHamburgers(name): 6 if name == "Alisher": 7 likes_hamburgers ="yes" 8 else: 9 likes_hamburgers="no" 10 return likes_hamburgers 11 12 print likesHamburgers("Bill") 13 print likesHamburgers("Alisher") 14 print likesHamburgers("foo")
Another classic example of a function
# # this is a simple function # def calculator(number1, number2): answer = number1 + number2 return answer print calculator(12,43) print calculator(91,673) print calculator(1,3) print calculator(87,1098)
Some decent videos about functions in Python