A lahar is an Indonesian word for hot or cold rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano and/or river valleys . When a lahar moves it look like a mass of wet concrete that carries rock debris ranging in size from clay to boulders more than 10m in diameter. They vary in size and speed and the smallest lahar is less that a few meters and several centimeters deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can flow several tens of meters per second - much to fast for people to outrun.
As a lahar rushes downstream from a volcano, its size, speed, and the amount of water and rock debris it carries constantly change. The beginning sure of water and rock debris often erodes rocks and vegetation's from the side of a volcano and along the river valley it enters. This initial flow can also incorporate water from melting snow and ice (if present) and the river it overruns. By eroding rock debris and incorporating additional water, lahars can easily grow to more than 10xs their initial size. but as a lahar moves farther away from a volcano, it will eventually begin to lose its heavy load of sediment and decrease in size.
What triggers these lahars can just be on eruption. More then one lahar can get triggered and by quickly melting snow and ice on a volcano or ejecting water from a crater lake. Mostly formed by intense rainfall during or after eruption. Stratovolcanoes are more common for a lahar to be on or near. This is because these consolidated rock debris that is easily eroded, or internally weekend by hot hypothermal fluids. Lahars are also common from the snow - and ice covered shield volcanoes in Iceland where eruptions of fluid basalt lava frequently occur beneath huge glaciers.
El Palmar, Guatemala on August 14, 1989 was a deadly lahar. there was white stream cloud uplift. People were watching everything get damaged from area up higher hoping they were safe. The lahar passed below the people with incredible speed and with an overwhelming roar. The flow was approximetely 5 m high and filled with the river channel. Small rock and mud splatter were thrown all over. The ground was vibrating and made it difficult to stand and a very loud roar.
There are many lahars after volcanoes and mostly with intense rainfall. Lahars are not a very difficult thing to understand just can be very unsafe.