"Rogue Waves" are waves that are much larger than other waves given the current sea state. The definition cited on wikipedia is: "[Rogue waves] are more precisely defined as waves whose height is more than twice the significant wave height (SWH), which is itself defined as the mean of the largest third of waves in a wave record."
Given the above definition, in relatively calm seas a rogue wave may really not be all that large. But rogue waves can be shockingly big in rough seas. And if you read the definition closely, it doesn't discount the possibility of a really big rogue wave in relatively calm waters.
It's worth noting that tsunamis are not rogue waves. The process that creates rogue waves is poorly understood, but we have an extremely clear idea on the causes of tsunamis.
From the above wikipedia article:
It is common for mid-ocean storm waves to reach 7 meters (23 ft) in height, and in extreme conditions such waves can reach heights of 15 meters (49 ft). However, for centuries maritime folklore told of the existence of vastly more massive waves — veritable monsters up to 30 meters (98 ft) in height (approximately the height of a 10-story building) — that could appear without warning in mid-ocean, against the prevailing current and wave direction, and often in perfectly clear weather. Such waves were said to consist of an almost vertical wall of water preceded by a trough so deep that it was referred to as a "hole in the sea"; a ship encountering a wave of such magnitude would be unlikely to survive the tremendous pressures of up to 980 kPa (142 psi) exerted by the weight of the breaking water, and would almost certainly be sunk in a matter of minutes.
Rogue waves are something that sailors have been describing for years (if not millenia), but scientists were hesitant to admit their existence until the so-called "Draupner wave" hit the Draupner oil platform, in the North Sea on January 1, 1995. Recording instruments on the oil platform measured the height of the "Draupner Wave". Damage to the oil platform itself confirmed the existence and size of the wave.
From a Fortean point of view, rogue waves are interesting in that they are an example of a widespread phenomenon -- supported by large numbers of eye-witness reports -- whose reality was rejected by the scientific establishment until it was impossible to continue denying its existence.
One of the better videos of a (relatively small) rogue wave impact was caught on a television show called "Deadliest Catch".