The Read-Evaluate-Print-Loop or REPL, is Python's command-line environment. To start the REPL type python in the terminal or command window. If you have multiple versions of python installed, type python2 or python3 or even something like python3.4 to start the appropriate REPL for the version you want.

Examples:
>>> 2 + 2
4
>>> a = 27
>>> a
27
>>> _   # The underscore is a special variable to refer to the most recently printed value ( only available in the REPL )
27
>>> print('Hello World!')   # print() is a function call in Python 3 and requires parenthesis. In Python 2, it does not.
Hello World!
>>> a = 'Hello World!'
>>> a
'Hello World!'
>>> print(a)
Hello World!
>>> for i in range(4):    # You can create code blocks which will execute once you exit the block
...        x = i * 2           # indent each level of code 4 spaces
...        print(x)
...
0
2
4
6
>>> def add_numbers(a,b):    # You can create functions in the REPL
...         print(a + b)
...
>>> add_numbers(10,30)    # Call your function
40
>>> help    # To get help, type help.
Type help() for interactive help, or help(object) for help about object.
>>> help()
help> q    # Type q to exit help 
>>>

To exit the REPL, use Ctrl+C on windows and Ctrl+D on Linux.