Because bitmap (.bmp) or "raster" graphics use a grid of dots (called "pixels") to make a picture, changing the image from its original size can make it begin to look fuzzy.  Any basic paint program can create, for example, a bitmap image of a balloon.  The program would generate the image you design by laying out different colored dots in a series of rows and columns, each with specific characteristics.  If you zoom in on any bitmap graphic, you will see that the tiny little individual dots blended into your normal size image become bigger dots.  More dots are not added to fill in the spaces, so the curves in the image start to look more jagged.  Bitmap graphics are not scalable.  You may have had the experience that your logo type looks blurry if a larger image is reduced to fit into a smaller place in an electronic publication.  The image was probably a bitmap.

Vector graphics use formulas instead of a grid to trace the outlines of the objects in an image and fill them in appropriately.  Once you finish your image, it is saved as mathematical formulas that describe precisely each object, its colors and placement.  The drawing is now scalable.  When you zoom out or in, the same formulas are used to produce a bigger or smaller version of the drawing by filling in the spaces, and the curves do not appear ragged.  Fonts can also be "vectored".  The two most common examples of scalable fonts are PostScript and TrueType.

If you stop to think about it, you will realize that vector graphics are easier on the computer (and make smaller images to store on a disk).  A bitmap must describe the color and placement of every dot used in the entire image and its background at a fixed size.  Bitmap files are typically very large.