Is PHP important to web design or is it just another back-end language [on hold]

I don't know if it is just another language you can choose for a backend language like python, or Java, or C#, or is it something essential like JavaScript. I already know a lot of Python and know quite a bit of C#. My questions I need answered are:

- What is a backend language and what does it do?
and
- Is PHP a backend language? If not what is it and what is it used for?