The main difference is that the output of pip freeze
can be dumped into a requirements.txt file and used later to re-construct the "frozen" environment.
In other words you can run:
pip freeze > frozen-requirements.txt
on one machine and then later on a different machine or on a clean environment you can do:
pip install -r frozen-requirements.txt
and you'll get the an identical environment with the exact same dependencies installed as you had in the original environment where you generated the frozen-requirements.txt.