When you give one of your project to a friend or colaborator, inside a compressed file, you have to tell him which dependencies to install to run the project. Fortunately virtualenv (actually pip) gives you and automated way to do it.
Suppose you have a project folder you want to export and suppose you have made a virtualenv for that project. With your virtualenv activated, run:
(env)dante@Camelot:~/project-directory$ pip freeze > requirements.txt
This will create a file called requirements.txt. In that file pip will place all packages names and versions installed for that virtualenv. Generated file (in our case requirements.txt) should be included in exported file bundle.
To import a project exported that way, importer should uncompress project folder and create a virtualenv in its location. With that virtualenv activated pip should be called this way:
(env)otherguy@host:~/project-directory$ pip install -r requirements.txt
This call to pip will install all packages and versions included in requirements.txt. Easy and efficient.