You could represent a letter as a set of pixels. Like this:
This is a picture of the letter E. I have defined a block of pixels, five by six, and defined some of those pixels to be dark and some to be white, to show the letter E. This is a raster font. If I want it bigger, or smaller, this little block of pixels will simply be magnified, or shrunk. It will look block if magnified, and when shrunk will lose detail.
Another way is to define the shape of the letter "E". I would say something like:
At the top of the letter space, draw a line from left to right.
Now draw a line from top left to bottom left.
Now draw a line from bottom left to bottom right.
Now draw a line from centre left to centre right.
Make each line thickness one-fifth of the total height.
When this letter is magnified or shrunk, it will not look blocky or lose detail as the raster version did.
Read the wiki page: http://en.wikipedia.org/wiki/Computer_font