Generators are a Python core language construct that allow a function's return value to behave as an iterator. A generator can allow more efficient memory usage by allocating and deallocating memory during the context of a large number of iterations. Generators are defined in PEP255 and included in the language as of Python 2.2 in 2001.
An introduction to Python generators by Intermediate Pythonista is a well done post with code examples.
This blog post entitled Python Generators specifically focuses on generating dictionaries. It provides a good introduction for those new to Python.
Python 201: An Intro to Generators is another short but informative read with example generators code.
Iterators & Generators provides code examples for these two constructs and some simple explanations for each one.
Python: Generators - How to use them and the benefits you receive is a screencast with code that walks through generators in Python.
The question to Understanding Generators in Python? on Stack Overflow has an impressive answer that clearly lays out the code and concepts involved with Python generators.
Generator Tricks for Systems Programmers provides code examples for using generators. The material was originally presented in a PyCon workshop for systems programmers but is relevant to all Python developers working to understand appropriate ways to use generators.