Overhang is a term used to describe the incorrect tracking path of the stylus for cartridges mounted in a pivoted tonearm. Overhang is not a problem with linear tracking arms, which track in a straight line directly across the record (the same path followed by the cutter head when LP masters are cut). Ideally, the stylus (in pivoted tonearms) ought to trace an arc that is tangential all the way across the LP, but that is not possible due to the relatively short length of the tonearm. Therefore, a stylus will only be "perfectly" tangential at two points along the arc it traces.
For the arc to be as accurate as possible, the cartridge must be mounted so the stylus will follow the desired arc. If the cartridge is mounted too far forward on the tonearm, however, the stylus will "overhang" the path of the desired arc. If the cartridge is mounted too far back on the tonearm, the stylus will fall short ("undershoot"?) of reaching the desired arc.
Hence, one of the main reasons for using a cartridge/stylus alignment device (or protractor) is to ensure that the cartridge does not "overhang" or "undershoot" the desired tracking arc. Getting the correct adjustment is not particularly difficult, although it can be tedious even with a good alignment tool.
There are several cartridge/stylus alignment tools that work reasonably well: the Dennesen "Soundtracktor" and the MoFi "GeoDisk" (both of which are no longer distributed, but can sometimes be found on the used market); the phono cartridge alignment tool sold by Turntable Basics (see this link for more info: http://www.turntablebasics.com/); and the rather expensive device called the Wallytracktor, which is custom made for the specific tonearm/cartridge setup that you own.
I have the original plastic version of the Dennesen "Soundtracktor" and it works quite well. If you can find one, buy it. If not, then the tool from Turntable Basics does an acceptable job.
