One may generate a requirements.txt via:
$ pip freeze > requirements.txt
A user can use this requirements.txt file to install all the dependencies. For instance:
$ pip install -r requirements.txt
The packages need to be in a specific format for pip to understand, such as:
# requirements.txt
feedparser==5.1.3
wsgiref==0.1.2
django==1.4.2
...
That is the “requirements format”.
Here, django==1.4.2 implies install django version 1.4.2 (even though the latest is 1.6.x).
If you do not specify ==1.4.2, the latest version available would be installed.
You can read more in “Virtualenv and pip Basics”,
and the official “Requirements File Format” documentation.