A digital image may be defined as a bitmap or a vector. A bitmap is a collection of pixels, for example a digital photo or a digital painting made in Photoshop. A vector defines a shape as a collection of points joined by lines (e.g. a square is four points).

Shown left in figure 1 is a close up of some text made in a screen-based Photoshop document, shown compared to a similar close up of text produced in Illustrator (right). See how the edges of the Illustrator text is cleaner than Photoshop's, which looks like a dog's dinner. This will remain true no matter how zoomed up the view is. Shown in Figure 2 right is a zoom of a vector form. Compare this to a zoom of a bitmap form on the left.

Figure 1. Text made in a screen-based Photoshop document (left), shown compared to text produced in Illustrator (right)

Figure 2. A zoom of a vector (left) compared to a zoom of a bitmap (right)

So... vector images rule, right? Wrong. A design such as a digital painting or a photograph is much better expressed as a bitmap. In Figure 3 is show a zoom of a bitmap image of a cat. See how the many nuanced colors are successfully expressed using thousands of pixels. In Figure 4 is the same photo that has been converted to a vector. See how simplified the colors are.

Figure 2. A zoom of a bitmap of a photo showing the individual pixels

Figure 3. A zoom of a photo converted to a vector, showing it to be made of individual shapes

Unlike Photoshop, which tries to serve (too) many communities, Illustrator serves (mostly) only two: graphic designers (which includes illustrators) and typographers.