Virtualenv is really a nice thing if you want to work with different python version.
suppose you have a project that need python 2.7 and in the mean time you also want to work on another project that need python 3.5. What can you do then? What is the solution for you?
sudo pip install virtualenv
Create a virtualenv for project
'env_name' virtualenv is create and you can use this now.
Install packages as you want in your virtualenv.
For deactivate this write