I'm pretty new to web developing and I'd love some clarification. Although reading more than one book on the topic, I cannot seem to wrap my head around the pixel concept. I encounter problems with this issue when trying to use CSS and pixel units for design that fits different screen sizes.

To my understanding a pixel is the most basic unit used by a monitor in order to compose an image on the screen. So if me resolution is 800 by 600, everything on my screen is rendered using those 800*600 basic building blocks. If I were to enlarge my screen resolution, 3 things would accrue:

A. The basic image building block(the pixel) would shrink in size B. The pixels would move close together C. Well, more pixels would now be available

All these combined lead to a sharper(depending on the viewing distance) and more detail enabling image.

Well so far so good. Here is were I start getting lost:

To my knowledge a pixel is not a physical, real object. Monitors are not embedded with a few thousand pixels. I am drawn to this conclusion because anyone can change his screen's resolution, making a pixel on his screen bigger or smaller, and adding or subtracting the amount of total pixels on screen.

Adding to that, I have herd that different monitors have different pixel densities. For example Apple's retina monitors.

Taking all of the above as my knowledge base, These are my questions:

1. If a pixel has no real world constant size, what does comparing different pixel densities matter? Each screen company can define it's own pixel concept and declare the higher density.

2. What does a bigger pixel density mean? Say we take two screens with the same physical dimensions, but with a different pixel density, am I to assert that the main difference would be the larger density screen being able to display a higher max resolution? Or am I to assert that given the same resolution on both monitors, the higher density one would display a sharper, smaller image?

3. If a pixel is not a fixed size within one monitor, is it a fixed size between the same resolution on two different monitors? For example, would two different monitors, set to the same resolution, be comprised of same size, same quantity pixels?

I'd love some help (: