r means read-only, b means binary.

Correspondingly, w means writable, and t means open in text mode.

Add some official explanations:

>>> print file.__doc__

file(name[, mode[, buffering]]) -> file object

Open a file. The mode can be 'r', 'w' or 'a' for reading (default),

writing or appending. The file will be created if it doesn't exist

when opened for writing or appending; it will be truncated when

opened for writing. Add a 'b' to the mode for binary files.

Add a '+' to the mode to allow simultaneous reading and writing.

If the buffering argument is given, 0 means unbuffered, 1 means line

buffered, and larger numbers specify the buffer size. The preferred way

to open a file is with the builtin open() function.

Add a 'U' to mode to open the file for input with universal newline

support. Any line ending in the input file will be seen as a '\n'

in Python. Also, a file so opened gains the attribute 'newlines' ;

the value for this attribute is one of None (no newline read yet),

'\r', '\n', '\r\n' or a tuple containing all the newline types seen.