Its a simple and interesting way of earning.All we have to do is register our mobile no. with these sites.They will send us ads to our mobiles based on the interests we had entered in our profile during registration.
For reading these simple ads we get paid
They have referral system too by which we earn for the ads received by our referrals
There are many sites offering such offers.
Given below are some of the most popular ones.
bravenet.com