If you want your program to get an input from the user (for example to enter some parameters for your program to use), we use the input("Message here") command, as follows:
number = input("Enter a number: ") username = raw_input("Enter your name: ")
When the program is run, this will show the user the message Enter your name: and will then save whatever the user inputs as the variable username. This can be very useful when writing a program to solve engineering type problems, as your program could ask the user to enter some parameters (such as mass, volume etc.) for your program to use.