The Net

The Net (commonly referred to as the Net or Cyberspace) is the name given to the vast telecommunications network created by the Human race.
It is analogous to the real world Internet, but much more extensive, including things like appliances and even cybernetic limbs. The Net is made of up of hard lines, radio links, cell networks, microwave transmitters and anything else that can transmit information from one computer to another.

Web of a Million Lies

The internet is effectively useless outside a few heavily-regulated regions, to the point where searches beyond those boundaries are best done with an accompanying exorcist. Adaptive botnets, long abandoned by their creators, flood the net with bullshit and targeted harassment.
The internet was a mistake. Everyone knows it, but we're stuck with it. The breakdown of reliable information transfer is a Great Filter - there are dozens of failed civilizations in human space as testament to that. The amount of art and records that have simply been lost to digital entropy is a very depressing number.
The passing centuries have made the net as dangerous to use as uncharted jungle is to walk through without boots, thanks to an overflowing ecosystem of constantly-adapting hostile data-life and the eternal shore of perniciously nasty humans.

Using the Internet

Access to the Net requires a modem of some kind. It is possible to use the Net the way we use the Internet in real life; with a computer or terminal, keyboard, and video display (also called Vidboards). But the professionals experience the Net in 3 dimensions, using a complex cybernetic interface called a cybermodem. People who use cybermodems are called Netrunners. A cybermodem provides an experience that is much more immersive and intuitive than the traditional keyboard/computer interface. This allows Netrunners to react far faster than would ever be possible with a keyboard.
Using the Net in anything more than the most rudimentary ways requires a cybermodem (sometimes called Cyberdecks) You do not necessarily need interface plugs installed in your head to use a cybermodem though. There are also options for electrodes or even 2D displays with a keyboard. But actual netrunners will always use plugs, because actual cybernetic connections wired to your brain ensure the very fastest reaction times. And in the Net, speed matters. A lot.
Just because you know how to surf the Net, doesn't mean you're an experienced diver in the fluctuating streams of pure, fluid data beaming directly into your brain.
In order to enter cyberspace, you need a SynthTech interface, external support (especially for the first time), a secure BBS and a lot of luck. Netrunners who can navigate the deep Net are mostly interested in accessing corporate data fortresses.
Unless there is something terribly wrong going on, players can always access the internet when in a station or colony that has a civilian population - military and corporate bases will have secure, pared-down networks that will have to be directly accessed via a terminal.
Basic usage includes things like looking up something on Space Wikipedia, hiring someone for a simple job (rideshares, etc), viewing a public personal page, making financial transactions with the Company, using a chat client, and so on. Players can just do this normally without incident.
Hacking a computer means getting access that you aren't supposed to have - getting into someone else's account, spoofing administrator privileges, and so on. This requires a skill check.
Venturing off the beaten track (weird imageboards, shady download sites, seemingly-abandoned conspiracy blogs, domain squatter hives) is dangerous: players should make a Sanity save (modified by a relevant computer or cultural skill).

Ghost Towns

Clandestine areas of the Net, known as "ghost towns", are hidden areas located throughout the Old Net, providing sanctuary to digitized consciousnesses. The central ghost town is the Ghost World, sometimes referred to as simply "Ghost Town", though almost all ghost towns are rumored to have been founded by the digital consciousness of Alt Cunningham.
The Ghost World is made up of a computer network based out of the ruins of Hong Kong that was abandoned during the desertion of the city, repurposed by Cunningham to be a place where all an Artificial Intelligence can live among their own kind.