So - a lahar is one of the effects of volcanic eruptions and is composed of water and pyroclastic material (very hot things, like lava, superheated water, and sometimes ash that didn't get high enough). It is a type of mudflow, which flows down the slope of a volcano. It is most common along the river valleys, as they get a proper course to follow. Lahars can be very destructive for the vegetation surrounding the volcanic mountain. They can become solid when the temperature lowers down, while in case of high temperatures they are extremely fluid. The origin of the term lahar is in the country of Indonesia where it first occurred.
Lahars can be very dangerous if they are of great density and speed and have the capacity to flow for a long distance and cover a large area. They can gather the momentum if they fall down a steep slope and travel above, and have been clocked at over 12 meters per second (27 miles per hour). One of the most devastating lahars in the recent past occurred in 1953 when the lahars from the Nevado Del Ruiz eruption in Colombia killed above 21,000 people.
A lahar can form during a pyroclastic flow if the snow and glaciers of the volcano's summit (which are very common for large volcanoes) are melted, results in lahars, the water picking up mud, and rocks, boulders, trees, and everything else that it can grab.
Flooding from glacier breakdown can also result in lahars, though not nearly as serious (usually) as a volcanically induced lahar.
Some very, very large and serious typhoons can drop enough rain in a short enough time to wash ash and rock that had been collecting on a volcanic mountaintop to create a lahar. This is generally the least damaging of the three types, because the fluid water doesn't usually pick up as much debris - on the other hand, they almost always travel farther than the other types.
There is no such thing as a good, safe, or small lahar, and certainly no one that you ever want to experience.
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