People are trying to earn money online for many reasons. Mostly women who hardly get time to work for a particular and fix time, are trying to work at home. Online work is local but connection is global. That is why women are usually interested in online jobs.
Some people who are enjoying a permanent job and earning a nice sum of money are also interested in online jobs like me. As they get enough time to perform extra job they try to earn extra money.
This jobs are of many kinds. Many jobs are available that is possible to be completed on odesk.com, freelancer.com, microworkers.com etc.
You need not have many skills for this. Just internet skills and a computer is enough to do such kink of jobs. For microworkers jobs you need to be expert in social book marking. For example facebook, twitter, myspace, delicious and other book marking sites.
Another source is Forum posting. Forum posting is very important factor for web traffic. A huge number of visitors gather on a forum and discuss here on various topics. This is the platform where you are both teacher and student. You may teach the subject in which you are expert and you may learn those subjects which is not clear to you. For thise reason you may have joined in thousands of forum. It will help you to work on microworkers.
Odesk is one of the largest online platform. You may also work here.