Microjobs can involve a huge range of different tasks. Some are quite fun, others are bitterly monotonous. One day you might be categorizing YouTube videos, the next writing short product descriptions, and the next recording your voice over and over again to help train Siri-like recognition systems. Many people look for online data entry jobs, and some microjobs are along those lines.
These little jobs are done by people who log on to a company’s site and choose tasks, which could be as simple as clicking a link. Amazon's Mechanical Turk is one of the most well-known sites of this type. Also, there are crowdsourcing projects, which are similar to data entry, where companies engage an army of virtual workers to each do one small part of a larger project.