The light from a laser is coherent, all the waves of the same wavelength, color, and all in exact lockstep with one another. When the beam strikes a surface and bounces off and the wavefronts collide with one another and can reinforce and/or cancel one another.
The [sparkle pattern](https://en.wikipedia.org/wiki/Speckle_pattern
) of a laser can be compared to the choppy surface of a lake on a windy day as waves cancel and reinforce one another to the extent that no regular pattern of waves can be seen.
The main concept behind this is Interference. This specific type of interference is known as speckle effect.
Interference can occur when you have a coherent light source(same wavelength, same waveform, constant phase difference). It can be either constructive or destructive depending on the phase difference between two waves.
This phase difference can be caused by a difference in path length. Surfaces, no matter how polished they appear to us, aren't completely flat, so the Laser's reflected light(interpret these as being a multitude of waves) will have some path length differences, which will interfere with one another, giving you spots with destructive interference and spots with constructive interference, which appears grainy to your eye.
This is as concise as it can be without removing too much information.
* By the way, *fluorescence* generated by the Laser will not be coherent, so if you want to see the necessity of coherence to generate interference, just shine a laser with a wavelength short enough to cause fluorescence on an object (a 405nm laser on a white piece of paper will do), and you will see that the resultant light has no speckle effect whatsoever.
What you are referring to is probably laser speckle.
I can't provide an amazing explanation off the top of my head, but i'll try for something simple.
It all depends on the surface. The surface of a wall is uneven, so when a laser shines onto a wall and into your eye, the sections of light are all travelling slightly different distances. Because of these slightly different distances, our eyes see bright and dark spots, which are the result of constructive and destructive interference.
We only get this interference effect because laser light is (to put it simply) a consistent colour (wavelength).
Speckle Effect. Black spots are where a crest and trough hit and "cancel" out. It just opposite phases of the light wavelength. Since all the light coming out of the diode is the same wavelength you'll get this as the effect.