Update itertools.md

Author: Joseph Luce

Diff for: pythonic/itertools.md

@@ -1,6 +1,21 @@
 # Itertools
 This section will go over a few tools in python 3's itertools library.
 
+## Iterables vs. Iterators
+
+Before we begin, you can't understand itertools unless you understand iterators and iterables.
+
+### Iterables
+
+### Iterators
+Iterators is an object which can iterate over an iterable. 
+You can create an iterator with the **iter()** method. 
+You can iterate by implementing the **__next__()** method, this returns the next item in the object.
+
+When you use a for loop, you are actually using an iterator. An iterator is created for you and loop across your iterable.
+When an iterator reaches the end of the list, it will automatically throw a StopIteration exception.
+The for loop catches that exception and exits gracefully.
+
 ## count()
 
 ## cycle()