What is clinical depression and how is it diagnosed?

First off, just in case anyone is offended, I apologise. I’m just not really sure how it works and I’m sure I couldn’t possibly know unless I experience it but I’m just wondering about it, and thought that I’d be much better off asking intelligent Flutherites rather than perusing shady “medical” websites.

What makes depression clinical or a mental disorder rather than just being down and gloomy?

Does it have to last for a certain amount of time? Do there have to be physical symptoms as well?

How could one possibly know if they were clinically depressed? Do you book yourself in to a doctor because you’re worried about yourself or does it get to a stage where someone else has to book you in?

Ok, I admit I did google first and came across this and I’m probably experiencing about 15 of those symptoms, yet I hardly think I’m clinically depressed. I know the internet isn’t to be trusted on these issues, but really, what makes someone clinically depressed?

Sorry again, I know this is a tough subject, but I’m just trying to learn.