[Python] What are the differences between Set, FrozenSet, MutableSet and AbstractSet in python...

I am trying to annotate my code with types but I am a little confused when it comes to sets. I read some points in PEP 484:

Note: Dict , List , Set and FrozenSet are mainly useful for annotating return values. For arguments, prefer the abstract collection types defined below, e.g. Mapping , Sequence or AbstractSet .
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and

Set, renamed to AbstractSet . This name change was required because Set in the typing module means set() with generics.
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but this does not help.

My first question is: what are the commonalities and differences between Set, FrozenSet, MutableSet and AbstractSet?

My second question is: why if I try

from collections import FrozenSet


I get

ImportError: cannot import name 'FrozenSet'


?

I am using Python 3.4 and I have installed mypy-lang via pip.

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