Tonearm/cartridge weight considerations


I have just decided to dig up my old vinyl collection that has been waiting for better days since mid-80s. Bought a turntable (Thorens 124) without tonearm. I have Shure V15V-MR cartridge. Started to look around for an appropriate tonearm and found out that the tonearm/cartridge issue has grown into a science. Now I am confused about this tonearm/cartridge weight issue. Could enyone illuminate me about this issue, mainly covering the following questions:
1) they speak of light/medium/heavy arms, so what are the figures for those weight categories?
2) are there weight categories for for cartridges and if yes - where my SHure V15V-MR fits?
3) does a light tonearm needs a light cartridge, or is it totally opposite - a light tonearm needs a heavier cartridge and vice-versa?
4) what can go wrong if the rules of tonearm/cartridge weight are ignored?
Thank you!
crockusbird
Actually you are trying to match the effective mass(different from weight) of the tonearm and compliance of the cartridge. You've chosen a very easy to match cartridge in the Shure, being a medium compliance piece. Look at this page for more insights-(http://www.theanalogdept.com/cartridge___arm_matching.htm).
I would consider the Shure a high not medium compliance cartridge.

At 25 it is far above what generally is accepted medium i.e. 12-17.
I suppose I missed the fact that there was no, "X" in his designation. The V15VX-MR has a compliance of 15. My bad! In that case: look for a fairly low effective mass arm. My Denon 103D's compliance is 30, and works wonderfully with my old Magnepan Unitrac I.