Backlink Query

Dear members,

I have a question about backlinking. I am researching on the how's and what's of backlinking, and also checking the backlinks of other top listed websites to get an idea of who to backlink with. Well what a surprise !! One particular site which sells weight loss products, had over 500 backlinks to a voucher website. When I looked at the backlinks, the pages had nothing to do with weight loss, actually it was anything but weight loss (clothes, dvds, finance, food, etc). I don't understand . How can a website be so backlinked, be on the first page of Google, but have backlinks that have nothing to do with its product? I tried looking for the link back to the weight loss page, and there wasn't any. Are they using a 'hidden' technique of backlinking? Have I stumbled across something which only the 'gurus' know? I am not a daft person and the warrior forum has saved me from buying the 'buy me, I can make to squillions' products. I am also learning the hard and long way of trying to make some money off the Internet, I have made a bit here and there, but only a bit here and there . So back to my question.....how are these websites able to backlink to websites that have nothing to do with their products and have no active link back to their page? Am I missing something or just completely misunderstanding the concept of backlinking ?? Thank you. Any replies would be appreciated. Afia.