An Elementary Encyclopedia of Experimental Electronica

Experimental music, being largely free of the confines of record label contracts and marketing appeal, tends to be very grassroots and artist-based. As a result, it's incredibly common for artists to coin a term for their own sound, which rapidly passes into common usage as it influences other musicians. This has led to an almost absurd plethora of nested styles, genres, and subgenres that renders any attempt at a comprehensive list almost inherently impossible, as new forms develop almost monthly. However, a few genres form the backbone of most experimental music, most of them centered around a principle technique or concept (excessive sampling, silence, distortion) and allowing plenty of freedom with the actual sound or mood evoked.

Tape Music 
Tape music describes a wide variety of styles making use of cassette tapes. This often involves physical cutting and pasting of tape segments, the deliberate misuse of cassette decks, the creation of seconds-long tape loops, and oftentimes makes use of elaborate home laboratories devoted to squeezing a whole world of sounds out of circuitbent hardware and winding reels of decaying tape. Negativland, showcased below, has been "culture jamming" since the 80s by irreverently recycling and combining tapes of everything from news reports and rock songs to phone calls illegally snatched off the airwaves. 
Example: Negativland -

Turntable music
Similar to tape music, here turntables are used as an instrument to reinterpret abandoned media: smashing 78s them and pasting them back together into Frankenstein records, changing playback speed and direction to bring them into new light, and modifying records to create surreal loops on the fly. The Caretaker makes use of forgotten records by turning them into dusty, sentimental vignettes and bleak illustrations of decaying memories.
Example: The Caretaker - An empty bliss beyond this World

Sound Collage
Sound collage ranges from the peaceful combination of juxtaposing sounds to wild mosaics of tiny shards of mashed-up media, but always makes use of unlikely combinations and cut-n-paste techniques in much the same way as a physical collage. Chuck Person collects all the flotsam of the 80s, home movies, computer ads, every scrap of stray nostalgia and compacts into into an unrelenting pastiche of cultural debris.
Example: Chuck Persons - A.D.D. Complete

Electroacoustic, as the name implies, finds unity and common ground between electronic synthesis and modification and acoustic real-world sound sources. This physical-digital peace treaty often warps conventional instruments into strange versions of themselves or creates instruments out of unlikely things, such as a pair of tea cups.
Example: Steve Roden - Winter Couplet

Less is more, and reductionists make use of this by carefully using silence as much as they use sound itself, spacing out noises and holding extended notes to create wide-open aural spaces.
Example: Toshimaru Nakamura - No-Input Mixing Board

Free Improvisation
Anything goes in free improvisations. Borne from the groundbreaking philosophy of free jazz's unpredictability, free improv takes things one step further and 
Example: 7FORM Troupe of Magical Performing Arts - 002

Never has a genre been as plagued by court cases, copyright strikes, and overall censorship as plunderphonics, a style that advocates the use of sampling as an instrument in it's own right, reinventing old sounds in wild new ways. Radio shows, rock songs, classical music, pop culture, and every other sound under the sun goes into the plunderphonics blender and comes out as a subversive
Example: The Avalances - Frontier Psychiatrist

Noise artists give unwanted sounds a home by embracing distortion, breakage, chaos, and cacaphony, creating impenetrable walls of, well, noise to bash even the heaviest rock or metal senseless. In spite of it's chaotic and even dangerous nature - one infamous noise show involved the driving of a bulldozer through the back wall of the venue - many artists find room for melody and harmony, buried in the background or playing right alongside blasted-out feedback and overloaded systems.
Example: 光の緒 - Untitled

Upon discovering the beauty of diatoms (tiny single-celled organisms) under the microscope, some scientists made a painstaking practice of arranging them into borderline-microscopic works of art. Microsound is the audio equivalent of this: millisecond clips of sound are collected and meticiously organized in place of, or in conjunction with, conventional beats or notes.
Example: Alva Noto - Prototype 2

As technology and it's consequences increasingly proliferated, an underground formed that, instead of rejecting it, embraced it as an artistisic ideal. Like most of these, industrial is self-explanatory, taking heavy influence from man-made wastes, factories, and machines.
Example: Ramleh - Hole in the Heart

What's the fun in life if you know exactly how everything'll pan out? Indeterminacy makes full use of this idea by using live radio feeds, unpredictable playing methods, and sometimes even animals to bring untamed unknowns to play.
Example: Igorrr - My Chicken's Symphony

Just as industrial formed as a response to urbanisation and pollution, glitch is a reaction to our ever increasingly digital lifestyles, finding peculiar beauty and humanity in the most inhuma places. Tones, beeps, buzzes, hisses, sounds at the very edge of human hearing or even totally beyond it all come into play.
Example: Ryoji Ikeda - data.matrix

Musique concrète

Much of experimental music works by taking one or more key elements away from conventional music and playing with what's left. Drone focuses on long, unchanging sound, removing rhythm completely from the picture and instead having timbre and soundscaping take center stage. These pieces can be extraordinarily long and are often unmatched as a calming aid to meditation. Plenty of other artists, however, use darker drones to unsettling and dread-inducing effect.
Example: Bull of Heaven - 45: The Wicked Cease From Struggling

Sonification artists seek to find the beauty in raw data by converting everything from the human DNA sequence to earthquake readouts to today's weather forecast into audible sound. The results may be cacaphonic walls of alien sound, revealing the strange inner working of digital compression and storage, or eerily beautiful, not unlike taking a microscope or blacklight to the everyday world and seeing it in a new way.
Example: TCLB - Weather (Temperature, Rain, And Wind Speed Averages): 1:00 AM 6/10/2015 To 10:00 PM 6/10/2015