Vector images, when applied to creating images with computers, are produced using points and mathematical expressions which define how each line gets from one point to the next. These points also define attributes such as stroke thickness, color, fill, and other properties. Because vector art is defined mathematically, unlike raster images1, scaling has no effect on resolution when printing, rendering, or viewing on a computer screen. Adobe Illustrator and Inkscape are examples of vector based drawing programs. Vector base drawing is used in animation software such as Moho and Synfig as well.
I enlarged the vector image, saved it, then cropped it to match the size of the first image. As you can see, enlarging the image had no effect on its quality. Note that these are rasterized images of the original vector image, and so will also degrade if enlarged beyond 100% of their size. Also, they may not look as sharp if you view them too close to the screen.