In Django filter statement what’s the difference between __exact and equal sign (=)?

There is no difference, the second one implies using the __exact.

From the documentation:

For example, the following two statements are equivalent:
>>> Blog.objects.get(id__exact=14)  # Explicit form
>>> Blog.objects.get(id=14)         
# __exact is implied This is for convenience, because exact 
# lookups are the common case.