from __future__ import print_function

def main():
    a = raw_input('First number: ')
    b = raw_input('Second number: ')
    print(a + b)

First number: 2
Second number: 3

When reading from the command line using raw_input(), the resulting value is a string. Even if you only typed in digits. Therefore the addition operator + concatenates the strings.

Remember, in Python 3 we would write input() instead of raw_input()