A requirement file is a simple text file with a list of Python packages. These List is usefull to install all needed packages with one pip command.
In PyLucid we use this requirements files to store the dependency information, with a "version range". e.g.: Django>=1.3,<1.4 (To get Django v1.3.x)
We use this information for building and upgrading the virtualenv, see also:
more information about pip:
django-processinfo: 5.4 ms of 217.8 ms (2.5%)