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 % operator.

name = 'marcog'
number = 42
print '%s %d' % (name, number)

will print marcog 42. Note that name is a string (%s) and number is an integer (%d for decimal).

See https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting for details.

In Python 3 the example would be:

print('%s %d' % (name, number))

The "%" operator is used to format a set of variables enclosed in a tuple (a fixed size list), together with a format string.

print ("%d squared is %d" % (10, 10*10))

The % operators in the string are replaced in order by the elements in the tuple.

%d is used for integers, where as %s is used for strings

Eg.

>>>name = "John"
>>>age = 23
>>>print "%s is %d years old." % (name, age)
John is 23 years old.

Python copied the C formatting instructions.

For output, %i and %d are the exact same thing, both in Python and in C.

The difference lies in what these do when you use them to parse input, in C by using the scanf() function. See Difference between format specifiers %i and %d in printf.

Python doesn't have a scanf equivalent, but the Python string formatting operations retained the two options to remain compatible with C.

The new str.format() and format() format specification mini-language dropped support for i and stuck with d only.