On some you get awarded points, but you need loads of points to actually get anything useful. Most of the cash ones require filling in a helluva lot of surveys, for very little money.
I signed up for several as someone told me they got well paid for doing them but half of them I didn't qualify for (wrong sex, didn't use the product, too old, too young - you get the picture) and the other half took so long to complete I lost the will to live.
I am with about ten different companies and some are far better than others, some send you a cheque when you reach a target. Others you can redeem for high street vouchers. There are also some that let you test products and pay for your opinion is a pin money earner, though one I use is to scan your shopping when you get home and that has earned me quite a bit over the years,
You do of course have to reveal a lot of details about yourself, so be sure they are trustworthy companies.