Cyberpunk was the unholy spawn caused by a bad bag of mushrooms and a rather lukewarm reaction to Lord of the Rings slashfiction. The general point of all cyberpunk is that evil multinationals have taken over the world, and only a handful of hackers and/or AIs that may or may not want to destroy all humanity can save the day. They do this by entering the most embarrassingly bad vision of the Internets ever.
Cyberpunk internets are almost complete quasi-physical constructions, which means something as simple as following a link or looking into a new folder means literally walking from one end of a virtual reality to another. To upload or download files, they have to make their avatars literally drop or pick up files inside giant cities, when FTPs or web browsers can do the same thing in seconds. TECHNOLOGY!!!
To cover up for the fact that a sci-fi book using essentially real-world technology only a few decades more advanced than ours can somehow fail to grasp even the most basic of computer workings, they try to add something mystical or religious. For example: "zOMG!!! The corporation is Babylon, and the computers are Zion!"
Cyberpunk is not just a genre of books; it is a lifestyle. Just like being furry or gay is a lifestyle. The majority of cyberpunk kids grew up in homes wallowing in the fat of their buttertroll parents and, unable to fight for themselves, they turned to the Internets and became trolls.
- Steampunk is when you have steam powered mecha like in that Will Smith movie. It's generally very stupid because, during the time frame in question, England was the United States and the United States was all about cowboys fighting Indians. The critical consensus in most circles suggests that contemporary steampunk literature is built around the use of steam and hydraulic action as a metaphor for homosexual attraction, what with the pumping, spraying, heat, and high-pitched noises. Steampunk is also interesting for having spawned a particularly special genre of music that would sound passable, if the lyrics weren't so stunningly fucking stupid (see Abney Park).
- Cthulhupunk is the highly improbable genre in which you get nerds on the Internets worshiping H.P. Lovecraft, and somehow they aren't just being fat losers but, like, it's all real. Tentacle rape is often involved.