Mass wasting is simply things falling down. Specifically, geological things falling down. Mass wasting, AKA mass movement or slope movement, is simply rocks and dirt moving downwards as gravity pulls on them. This includes sudden collapses and slides, and gradual downwards migrations that can take centuries. It is wasting away of mountains and hills as they loose mass. It is the process of the land settling.
The primary force behind mass wasting is gravity. Weathering (the effects of wind, flowing water, ice, and air) aids and abets mass wasting, but is considered a different agency. Any given slope (or cliff) has forces trying to keep it standing -- internal friction between the grains of sediment in dirt and interlocking mineral crystals within rock. Plant roots and small amounts of water (with its attendant surface tension) also strengthen the cohesion of earthen banks. These factors determine a slope's shear strength.
Shear stress can overcome shear strength for many reasons; saturation with water, frost wedging, earthquakes, plant and animal activity, chemical decomposition and disintegration of stone, etc. Mass wasting has many different forms, from avalanches to the invisible creep of soil towards the sea. Here are some of the main types of mass wasting.
Falls are quite simple; a rock or other mass falls down a cliff or steep slope at a high speed. Often the rocks will fragment when impacting the base of the slope, resulting in talus. Falls have a number of causes -- frost wedging (ice expanding in cracks in the rock), earthquakes, tree roots weakening cliffs, chemical decomposition and disintegration of stone, rocks being undercut by weathering (particularly by running water and waves), and dynamite.
Rock avalanches may begin as falls, when a large mass breaks off and falls down the slope. As the mass breaks up and tumbles down the slope, the avalanche slows and looses energy until it turns into a flow (see below).
A slide is a large mass of rocks and/or sediment that moves down a slope in a coherent block. Masses that start as a slide may break apart and become a flow, particularly the lower part of the mass, when the slide hits a projection or a sudden decrease in slope.
Rock slides occur when a large section of rock slides down a mountain; this is distinguished from an avalanche, because the rock moves as a single unit, as opposed to a tumbling field of broken rocks. This does not mean that they are small. Entire mountainsides can slide downslope as a single unit. Rock slides most often occur when there is a plane of weakness between layers of strata, a plane of cleavage within a metamorphic rock, or a joint or fault in the rock. They may be triggered by an earthquake, by chemical decomposition of a lower strata, or the slope becoming saturated by water, which can increase the weight of an over-laying strata, or saturate an underlying strata of shale or clay, making it 'slippery'.
Slumps are sometimes lumped in with slides, as they are very similar. While slumps also involve a large mass sliding in a unified unit, they don't necessarily break along pre-existing fractures or sedimentary layers. A slump occurs when something (most often water or human construction, such as road construction) cuts away at the base of the slope. Note that the water doesn't need to undercut the slope, it just amputates it.
Think of a pile of dry sand; if you carefully scoop away a bit of sand from the edge of the pile, some more sand will fall down, creating a new slope. While sand will flow down (each grain moving independently of the others), rather than slump, the theory is the same. Many materials cannot support a steep slope. Thus, when enough material is removed from down slope, an undersupported section will slide down until it reaches a new equilibrium. If there are no pre-existing fractures, the slide will create one, a concave slice, and the slump will have some rotational movement. This usually results in one or more flat-topped but angled scarps. (A diagram* is very useful to help visualize this).
Perhaps the clearest example of granular flows is dry sand. Sand rarely has cause to fall or slide as a cohesive unit. When sand moves, it is each grain for itself. Sand, once it is in motion, moves more like a fluid than a solid. Given enough kinetic energy even boulders can act like sand, each bouncing along on it's own individual trajectory.
Because of this, avalanches, are usually considered a type of granular flow. Once an avalanche gets going at a high velocity, it can travel for long distances down slope, picking up new debris as it moves along.
Not all granular flow is violent, however. Creep happens very slowly, the exact speed dependent on the slope and the materials that are creeping. The motive forces behind creep are often the same forces that cause weathering, although creep refers to the slow downhill movement of particles, not the slow disintegration of geological formations (yes, the terminology has some wiggle room here; a hill will slowly be 'weathered' away through creep, and the creep is caused by weathering).
The time frame in which creep becomes evident may a matter of months, or decades, or even longer. The wetting and drying of soil, or freezing and thawing, burrowing animals, tree roots growing -- all of these cause a slow, random churning of the soil and smaller rocks. But, of course, gravity makes sure the movement isn't completely random, as downslope movement will always be more likely than upslope. As the smaller particles are moved downslope, the larger stones and boulders resting on them will also slowly move downhill (or move downhill quite quickly, after they have been undermined sufficiently).
Slurry flows are flows that contain a lot of water (about 20% to 40% water). The causes of slurry flows are pretty obvious; heavy rains and sudden floods.
Mudflows are the obvious example of a slurry flow. If you add enough water to dirt, it will start moving downhill. Note that dirt can start flowing before water saturation reaches 20%; if the flow contains less, it is technically called an earthflow. Mudflows can be very fluid, and move at high velocity. They often follow the course of a pre-existing stream or watercourse, but due to their high speed, swollen mass, and high density, they can be quite destructive.
Debris flows are the watery equivalent of avalanches, and can in fact develop from avalanches. They are thundering masses of rock and other debris, lubricated with a good quantity of water. Or, alternatively, they are leisurely flows of meandering rock and dirt. The speed of debris flows can vary from 1 meter per year to 100 meters an hour. Like mudslides, they tend to follow pre-existing watercourses.
Ash-filled mudflows and debris flows of volcanic origin are called lahars, and they often tend to be quite sudden, and hot.
Solifluction is the wetter version of creep. When surface material is saturated with water for long periods of time, it flows downhill as water is wont to do, albeit much, much more slowly. This is most common in areas where water cannot drain, for example in areas with a layer of permafrost that prevents water from soaking down into the ground.
*http://seis.natsci.csulb.edu/bperry/Mass%20Wasting/Types_of_Mass_Wasting.htm (Photos and diagrams)