Someone who works on the computer, especially hacking that hacks into systems not to look for anomalies or to destroy a system. They're basically in there screwing around causing minor or no damage.
A grey hat is some one with a mixture of black and white hat experience. Usually, a grey hat is someone who performs network security or system security analysis for organizations.
BOB the grey hat will scan your webserver for vulnerabilities for x amount of $. When he gets home he will reverse engineer microsoft products using stolen windows code.