Explaining Python's '__enter__' and '__exit__'

I saw this in someone's code. What does it mean?

    def __enter__(self):
        return self

    def __exit__(self, type, value, tb):
        self.stream.close()

Here is the complete code.

from __future__ import with_statement#for python2.5 

class a(object):
    def __enter__(self):
        print 'sss'
        return 'sss111'
    def __exit__(self ,type, value, traceback):
        print 'ok'
        return False

with a() as s:
    print s
    
    
print s