Cyberpunk is a genre based on the exploitation of cutting-edge electronics technology by human society to create Grimdark. The setting takes place in big-ass futuristic cities that are extremely shitty places to live, what with 80% being ghettoes and industrial sectors. The characters in a cyberpunk are technophiles who are always at least 30% metal and cybernetics/bionics. Some of the most common themes in cyberpunk are gangs of "wired" thugs who use their built in advantages to mug people, corporations who literally own the government and everything, vigilante hackers who stumble across engineered viruses made by authoritarian governments to wipe out people they don't like, and there's at least one scene with a fat guy wearing a VR set having sex with a robot or something. Essentially, Cyberpunk is a deconstruction of the "traditional science fiction" of the 50's and 60's, especially its depiction of futuristic (often utopian) societies. Cyberpunk posits that Technology doesn't automatically make society better, because people will still treat other people like crap, and ever-advancing tech will become just another means of obtaining an advantage or fulfilling one's self-indulgences.
Notable Cyberpunk traditional games include Shadowrun, Cyberpunk 2020 and Netrunner. GURPS Cyberpunk also exists, and the house of the writer for it was infamously raided by the FBI. Cyberpunk has gotten popular again due primarily to a recent rise in cyberpunk video games: particularly Deus Ex Human Revolution, but upcoming Cyberpunk 2077 and Net Runner (made by a fa/tg/uy) also look liable to introduce the genre into the sphere of computer-aided gaming.
Many recent works colloquially known as Cyberpunk are actually what is known as "Post-Cyberpunk", a pseudo-genre which plays down the Asian themes (as its kinda hard to see Japan taking over everything after its economy crashed in the 90's and sociopolitical issues sent its birth rate on a downward sprial) and grimdark a bit (i.e. your boss in Deus Ex HR at one point orders you to help him save wounded co-workers first; contrast with the normal cyberpunk CEO screwing everyone else in order to save their ass). Calling something Post-Cyberpunk can be problematic depending on who you talk to, because it can be interpreted as a derogatory labeling. Where Cyberpunk is gritty and oppressive, Post-Cyberpunk takes the same advanced technology and socio-economo-political issues and runs with a relatively more lighthearted tone and setting. Some people see this as a betrayal of the whole point of the genre, while others see it as a more realistic interpretation of the same near-future world dominated by the effects of advancing technology and those who control it. Because the central difference between Cyberpunk and Post-Cyberpunk is tonality, /lit/ will fight forever and ever about the post-cyber/cyberpunk divide, asking "whether genre is defined solely on mood", or "if one man (William Gibson) can define a true genre", and "is Cyberpunk dead or are we just living it?".
On /tg/, we mostly just complain about Shadowrun advancing its canon.
Books what are Cyberpunkish
- Neuromancer by William Gibson
- The entire Sprawl Trilogy by William Gibson
- The entire Bridge Trilogy by William Gibson, but only sometimes
- Snow Crash by Neil Stephenson
- Do Androids Dream of Electric Sheep by Philip K. Dick
- A lot of things by Philip K. Dick
- Transmetropolitan by Warren Ellis and Darick Robertson
- Wikipedia entry on Cyberpunk
- Wikipedia entry on Post-Cyberpunk, within the larger "Cyberpunk" Derivatives page
- TVTropes entry on Cyberpunk