Back when I was designing and building websites for a living, I very often had to explain the concepts behind pictures to my clients so that they were best able to manage their own content.
I had to do it so often, in fact, that I created this article (originally posted in two parts,) and they all said that it helped.
We’re down the road a bit from those early internet times, but it may still have some helpful life in it yet.
Most modern documents have at least one embedded image. From Keynote and Powerpoint documents to websites and instruction manuals - they all have an image somewhere, even if it's just a logo. But as familiar as we all are to this "Ooh, look a picture" phenomenon, so many of us simply do not know anything about raster vs. vector image files or how to deal with them.
Diving Right In » Raster vs. Vector
All image files fit into one of two categories: Raster or Vector.
A raster image is the type of image that most people are most familiar with. It's made up of millions of colored dots that, if dense enough, form an image. It is easiest to see the 'dots' concept in old newspaper images, but all photographs are raster images whether analogue or digital, modern or vintage.1
The density of these dots is known as DPI or dots-per-inch, which works in the same way that your HDTV or computer monitor resolution works. The higher the number (i.e.: the denser the dots,) the higher the resolution of the image and the sharper and closer the image is to the true analogue: reality.
A vector image is a completely different concept. They are made up of strokes (lines,) fills, distances, and directions (hence, vectors,) rather than pixels or dots. Vector images don't have a DPI quotient as they have no dots. Rather, they have sets of coordinates with distances and color data embedded that make shapes. These shapes, when combined, form an image.