First, understand the difference between the two, raster and vector files that is. Raster files are what photos, scans, tif, etc are. A raster file is somewhat like having a whole gridwork of dots (ie pixels) on a page. You give a certain dot a color and when the dots are numerous enough and you are zoomed back far enough, you see a picture (drawing). To make a straight line in a raster image, you just make sure the dots are colored (black usually) and then are directly next to each other and inline. Well an AutoCAD drawing is a vector format file. Vectors are defined points (ie your line is drawn from point 0,0,0 to 100,0,0 and you now have a 100 unit long line). AutoCAD works by defining the vectors and then to represent a line on screen, it generates a colored straight line between those two points. To convert from raster to vector, you have to turn the lined up dots into a line defined by vectors. There are programs that will try to figure this out for you but it is not an exact science, and you will still need to "assist" the program to get a complete and final drawing. As an example, if you insert a raster image into AutoCAD and then try to "trace" lines over the raster lines, you will get what the raster-to-vector converters try to do. I know this is long winded but it might help to understand why converting from vector to raster is possible, but raster to vector is not quite so easily possible.
"Everybody is ignorant, only on different subjects." — Will Rogers
