A Virtual Environment is a python environment, that is an isolated working copy of Python which allows you to work on a specific project without affecting other projects
So basically it is a tool that enables multiple side-by-side installations of Python, one for each project.

Creating virtual environment in Linux

If pip is not in your system

$ sudo apt-get install python-pip

Then install virtualenv

$ pip install virtualenv

Now check your installation

$ virtualenv --version

Create a virtual environment now,

$ virtualenv virtualenv_name

After this command, a folder named virtualenv_name will be created. You can name anything to it. If you want to create a virtualenv for specific python version, type

$ virtualenv -p /usr/bin/python3 virtualenv_name

or

$ virtualenv -p /usr/bin/python2.7 virtualenv_name

Now at last we just need to activate it, using command

$ source virtualenv_name/bin/activate

Now you are in a Python virtual environment

You can deactivate using

$ deactivate

Creating Python virtualenv in Windows

If python is installed in your system, then pip comes in handy.
So simple steps are:

1) Install virtualenv using

 > pip install virtualenv 

2)Now in which ever directory you are, this line below will create a virtualenv there

 > virtualenv myenv

And here also you can name it anything.

3) Now if you are same directory then type,

 > myenv\Scripts\activate

You can explicitly specify your path too.

Similarly like Linux you can deactivate it like

$ deactivate