Magnitude is the measure of energy released by an earthquake while intensity is the measure of human reaction to the ground movement, and of the damage to the ground surface.
An earthquake is generally caused by the movement of crustal plates. And when the plates moved underwater it can cause large destructive waves called tsunami.
Recently, the earthquake in Japan registered in the Rossi-Forel scale with 8.9 intensity level. It was listed as the 7th strongest earthquake in the known history.
The Rossi-Forel scale of earthquake intensities is used to measure earthquake intensity while the Richter magnitude scale is used to measure earth magnitude.