They have many ways to earn bitcoin like doing signature campaign, by simple posting and gives some infornation you will be paid that. You can do trading also buy and sell that help to your bitcoin grow . If your bitcoin grow for sure you will earn money thats the simple explanation for that. But many people hold their bitcoin for few years so they can earn more profit ..
trading is better. There is no way that signature campaign can give you a good amount of money for a long time. SOon, they will end because there are too many spammers in this forum. Trading is very easy to learn. just spend about 2 weeks to a month, you can learn basic skills about trading. Time and practice will give you more experience and how to control your emotion during trading