If/Else Statements

I'm currently learning to program in python and I was wondering what is the correct syntax for comparing two strings.

I'm trying to compare the user input and decide whether it is a specific string so I could do certain calculations. Why does the following not work?

user_input = raw_input("Please enter a list of numbers: ")
user_input2 = raw_input("What do you want to calculate? ")
grades = user_input.split(',')

............
stuff
............

if user_input2 == 'sum'
    print grades_sum(grades)

elif user_input2 == 'avg'
    print grades_average(grades)

else
    print grades_std_deviation(variance)