What are unit tests and why should I care?

Question

Okay, I develop web applications in PHP and JavaScript and a lot of times here on Stack Overflow I have seen the word unit test passing by, but nowhere on the websi

Answer 1

On why unit testing should be done in the first place, I would like to add the argument by Barry Boehm: "a fault discovered and corrected in the unit testing phase is more than a hundred times cheaper than if it is done after delivery to the customer."

While the multiplier does sound a bit rough, I think the argument definitely justifies testing.