basically there both to do with your breaking performance, number one the rim grind, it is mainly for your pads to have a better bite so your brake becomes loud and more posotive when you pull the lever in, two tar is mainly used on a smooth rim or light grind, this is so your pads have something to stick to when you pull your lever in, tar is a very good thing to have handy with you at all times and may i add tar is not shit as said above, strongly disagree, although it dose built up crap on your pads, but it is easly clean of with red hot water, and a light sandpaper over the pad, but as long as you dont paint your rim black with your tar, you will be fine, and your brake will not stick......