Domain Flipping is the process of purchasing a domain, building a site on the domain, getting traffic to the new site and then selling the domain and website as a complete package. Keep in mind though that Domain Flipping takes time and energy and usually is not a great way to get rich quickly since there's no guarantee that your developed domain will sell. But with practice and patience it can become a very lucrative business. You should first join Namepros.com and DNforum.com to ask members about strategies and tips about how to flip domains.
Note: If you don't want to get through all the hassle then you can ofcourse just look for quality domain names to register, park the domains at Sedo and then try to sell them for a profit without doing any website development.