Tsunamis are waves generated by large earthquakes, underwater volcanic eruptions and even meteor impacts.

A tsunami travels at high speeds of up to 950km/h in the open ocean. This means that a tsunami wave might travel as fast as a jet airplane, although wave heights in the epicenter only rise one meter (3.2 feet) up.
The time period between tsunami waves is extremely long and ranges between several minutes and a few hours. The energy and power of a tsunami are, therefore, not comparable with a classic groundswell.
A tsunami wave is a force of Nature. As it reaches shallow waters near coastal regions, speed decreases and height increases (easily up to 30 meters/100 feet) causing flooding and devastation.
Tsunami wave are wrongly named tidal waves because tides don't play any role whatsoever. Tsunami Travel Times (TTT) software can calculate the arrival times of tsunamis, but it is not infallible due to bathymetry or/and epicenter estimation inaccuracy.
The example below shows the magnitude 7.2 Ms earthquake and consequent tsunami event hitting Hawaii on the 29th November, 1975. The colors tell us the arrival times of the infamous wave: red (1-4 hours), yellow (5-6 hours), green (7-14) and blue (15-21).
Hawaii, 29th November, 1975: a tsunami causes flooding and destruction | Illustration: NOAA
Around 80 percent of all "killer waves" are generated in Pacific Ocean's "Ring of Fire." A tsunami comes as a series of waves. Population must find protection in higher regions or nearby mountains. Tsunami means "harbor wave" in Japanese. Tsunami waves can't be surfed.