[Python] What's the difference between r'string' and normal 'string' in Python?

What's the difference between an r string (r'foobar') and a normal string ('foobar') in Python? Is r'string' a regex string?

I've tried the following and there aren't any effects on my regex matches:

>>> import re
>>> n = 3
>>> rgx = '(?=('+'\S'*n+'))'
>>> x = 'foobar'
>>> re.findall(rgx,x)
['foo', 'oob', 'oba', 'bar']
>>>
>>> rgx2 = r'(?=('+'\S'*n+'))'
>>> re.findall(rgx2,x)
['foo', 'oob', 'oba', 'bar']
>>>
>>> rgx3 = r'(?=(\S\S\S))'
>>> re.findall(rgx3,x)
['foo', 'oob', 'oba', 'bar']

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