There are two ways of expressing graphical images in the computer: Raster and Vector.

A raster image contains one bit of information for each "dot" of a picture.  A vector image contains a set of "points" that indicate where to draw lines.

Let's take this raster image for example:

Now, let's zoom in tight on the top of the tree:

Raster version:

Vector Version:

Both versions have their uses. Raster images can a bit more easily look like photographs, but Vector images can be zoomed in and out without losing detail.

In the 3d world, we NEED the vector version.