How does an eyeglass lens differ from a magnifying glass lens?

Very different. Eyeglass lenses can either magnify, minify (make images smaller) or correct astigmatism. Magnifying glasses are used in readers and prescription lenses for farsighted and presbyopic people, while minifying lenses are used for those with nearsightedness. Also, there can be magnification or minification with astigmatism correction at the same time. Glasses are built specifically for your vision.
Placement. Really no difference but how the magnifying glass is fitted to a frame, placed in front of your eyes and focuse to the retina of your eye (the back of the eye). Same principle but just different in where it is place and used. They both are focusing tools.