Ask HN: What are Unit Tests and Why are they important?

Okay, to say that I don't have the slightest idea what unit tests are may be a little misleading, but really, exactly what purpose do they serve? Is it kind of like Autoconf for php / ruby / whatever?

Do they test for things like XSS or just to see if you have GD installed on your server?

Is it terrible that I am not creating unit tests for in-house php based web apps?