Strings are everywhere. They can be as simple as 'Hello World' or as complicated as parsing an old-latin text and transforming it to unicode.

The basics can be found in the Basics chapter, from A Byte of Python.

Some more advanced examples can be found in the Strings chapter from Think Python.

And finally, if you want to dig even more, we have built our own Strings and Unicode guide which deals with complexities of Unicode and Byte strings.