They are used for formatting strings.
%s acts a placeholder for a string while
%d acts as a placeholder for a number. Their associated values are passed in via a tuple using the
name = 'marcog' number = 42 print '%s %d' % (name, number)
marcog 42. Note that name is a string (%s) and number is an integer (%d for decimal).
In Python 3 the example would be:
print('%s %d' % (name, number))