The term gank has several meanings, but often implies an overwhelmingly large group or much higher leveled player killing you and/or your group. ("A 60 Hordie ganked me"" or "Alliance players are ganking lowbies") It frequently carries the implication the attack is overwhelming or otherwise clearly biased against the individual, either being substantial lower in level or having been previously weakened by an unrelated threat.
Successful ganks usually consist of very short fights and depend on tactics like high burst DPS and the ability to hide; Rogues are stereotypical gankers. Areas such as "Ganklethorn Vale" and Hillsbrad Foothills are infamous for ganking, as they are locations that naturally overlap differently-leveled characters of opposing factions.