A domain investor, popularly known as domainer, is one who creates/buys domain names and registers them as a means of investment.
The domain investor may buy expired domains, trendy domains or they may just make them up. The domain investor may hold the domain for many years with the hope that the value will increase over time, much like a real estate investor would do. After some time, the domainer will sell the domain and hopefully reap profits.
This type of investment was made popular by Frank Schilling and/or Rick Schwartz. These investors accumulated many top level domain names and sell them to various start-ups and other agencies that yearn an internet presence.
A domainer may also be regarded as a speculative investor as they try to predict which domains will become popular in the future and invest accordingly.
The video below gives a short history of many popular domains that were sold for a lot of money (The Infographics Show):