What is the difference between pip and pip3?

These commands help us sort a little, because if we have more than one instance of python installed, the handling becomes confusing. For example in Ubuntu 14.04 pip is linked with Python2 and pip3 with Python3 [1].

Links:

1. https://www.quora.com/When-should-one-use-pip3-vs-pip-when-using-Python
2. https://www.quora.com/What-is-difference-between-pip-and-pip3
3. https://meta.stackoverflow.com/questions/361190/do-we-need-pip-and-pip3