Well, yes, you can earn money by clicking ads and by referring your friends to do the same thing. The Web has a lot of legit sites and more scam sites at the same time so be careful! And so I've listed some of the legit sites that I've found which is currently running for more than a year now and still stable but you should be aware that this does not necessarily mean that these sites won't turn into scam in the future; remember, ptc sites aren't scam free.
Joining PTC sites won't make you rich but who knows? Maybe just by clicking ads all through out the day and by referring people then you might get rich, right?! Well, kidding aside, it won't be really easy for you to earn big money here but it actually depends on how you do it and how persistent you are. Afterall, it might be such a grandiose idea but definitely not impossible. :)