Why do you need salt anyway?
The salt helps prevent the growth of harmful bacteria in the brine while also allowing the lactic acid bacteria to ferment the vegetables (through lacto-fermentation). There are some sites out there that advocate using little or no salt to increase 'diversity' of the fauna - I generally don't worry about this as the salinity level won't affect the bacteria we want. Also, the salt will help the cabbage (and any other vegetables) keep their 'crunch' even after several weeks of fermenting.
If you have searched on the Internet for information on this you will no doubt have come across a figure of 3 tbsp (tablespoons) of salt for every 5 pounds of cabbage. In metric this equates to approximately 1 tbsp of salt for every 750 grams of cabbage. Personally I find this level too salty and have found that one can make perfectly good sauerkraut using as little as 1 tbsp of salt for every kilogram (1,000 grams) of cabbage, especially if you use the brine preparation method outlined in the preparation article in this section (click here to view the preparation article). In short you can more or less salt to taste provided some salt is used and evidence of fermentation can be seen. Also be aware that unrefined sea salts are saltier than other varieties.
Also, the ratio applies to the total weight of vegetables - if you are making sauerkraut that has a significant amount (more than 10% of the total) of other vegetables (like carrots, fennel or kale for example) then increase the salt in proportion as if there were more cabbage.
I have found that the very best kinds of salt to use are Celtic or Pink Himalayan Sea salt, but any unrefined sea salt will do. I don't like to use any other kind, and haven't used any table salts to make my sauerkraut so I won't comment on their use here.
Do I really have to use salt?
Ok, fine - you can use very little or no salt and still get fermentation when using cabbage, as the bacteria we are after are still just as present and will still start to create the anaerobic environment which is hostile to more harmful bacteria.
However a word of caution: to do this I would advise using a starter culture, just to be sure, and keeping a good seal on and sterilising the container is all the more critical, especially in the first few days when it is easier for more undesirable types of bacteria to thrive.
Personally, I think using salt makes the fermentation process, taste and texture of the sauerkraut better - but you can experiment and see what you prefer.