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:
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.