What does the 'b' character do in front of a string literal?

Apparently, the following is the valid syntax:

b'The string'

I would like to know:

1. What does this b character in front of the string mean?
2. What are the effects of using it?
3. What are appropriate situations to use it?

I found a related question right here on SO, but that question is about PHP though, and it states the b is used to indicate the string is binary, as opposed to Unicode, which was needed for code to be compatible from version of PHP < 6, when migrating to PHP 6. I don't think this applies to Python.

I did find this documentation on the Python site about using a u character in the same syntax to specify a string as Unicode. Unfortunately, it doesn't mention the b character anywhere in that document.

Also, just out of curiosity, are there more symbols than the b and u that do other things?

To quote the Python 2.x documentation:

A prefix of 'b' or 'B' is ignored in Python 2; it indicates that the literal should become a bytes literal in Python 3 (e.g. when code is automatically converted with 2to3). A 'u' or 'b' prefix may be followed by an 'r' prefix.

The Python 3 documentation states:

Bytes literals are always prefixed with 'b' or 'B'; they produce an instance of the bytes type instead of the str type. They may only contain ASCII characters; bytes with a numeric value of 128 or greater must be expressed with escapes.