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!