So flash to present day and we all are using computers, and nothing is more apparent than the way we visualize the space between our screen. Here is were pixels come in, a sort of digital rule of thumb for anything from geographic images to 1 pixel wide background images use ubiqitusly across the internet.
The pixel is the most common form of visual computer measurement.
What size is your picture? 400px by 300px. How many pixels is the gap between your eyes? 2px. Your image cannot be larger than 1024px by 1024px.
These are just a few examples of how and what we run into on our adventures through the internet and social connections. All of these require a base knowledge of pixels and why they are used so much.
A detailed history and brief on pixel can be picked up at your local WIKIPEDIA.