Hacked login details. Cybersecurity exploits for hire. Drugs, guns and ammo. If there's something shady going on online, chances are it's happening on the dark web.

When Target was hacked in 2013, customer card details turned up on dark web marketplaces. Hackers have tried to do the same with Yahoo login credentials, and details of O2 phone network customers in the UK.

You'll also find cybercriminals selling security exploits. Ransomware, anyone?

Everything's for sale if you look in the right place. And with the rise of bitcoin, the "currency of choice" on the dark web, virtually anonymous payments are easier than ever.

Just this week in Australia, a news investigation revealed that an anonymous dark web user has offered up access to the Medicare records of "any Australian" for just 0.0089 bitcoin ($22, AU$30, £18).

That's not to mention the things you really don't want to see. Europol says the dark web and other peer-to-peer networks are still the "main platform" for sharing child abuse material.

So for those of us used to opening Chrome or Safari to get online, the dark web is an entirely different beast. How does it work? How is it different from the "surface web" that we all know? And what do you need to know ahead of time, should you choose to wade in?

The first thing to remember: The dark web is not the same as the "deep web."

The deep web refers to any part of the internet that isn't discoverable by a search engine. But that doesn't mean it's suspicious -- there are plenty of sites you visit in your day-to-day browsing that fall into this category.

When you log in to internet banking, you've navigated to a specific location online, but one that's not served up in Google results. The same goes for the different pages that pop up in webmail services, like Gmail, or academic databases on a university network.

It's hard to estimate just how big the deep web is, but the commonly cited research (albeit from 2001) puts the deep web at 400 to 550 times the size of the "surface web."

If the surface web is the tip of the iceberg and the deep web is what's below the water, then the dark web is what you'll find deep in the blackest waters below. The darknet refers to the peer-to-peer network itself, whereas the dark web is the content that is served up on these networks. 

This is where you'll find the kind of marketplaces that ply their trade in illicit wares -- what security researcher Brian Krebs calls the "hidden crime bazaars that can only be accessed through special software that obscures one's true location online."

The UN noted last month that although drug trafficking over the dark web is relatively modest, drug transactions increased 50 percent annually from September 2013 to January 2016. And in early 2016, then-US Attorney General Loretta Lynch warned that some gun sales were shifting to the dark web to stay outside the reach of regulations.

Anonymity is the key here. Whistleblowers, activists and political dissidents certainly have good reason to obscure their online location and post with anonymity on the dark web, but that level of secrecy is also sought by criminals.

This isn't just a matter of heading to "darkweb.com" and having a snoop -- you'll need specific software and a dedicated browser. The Tor software (and its dedicated Tor Browser) is probably the most famous of these, though there are others, including I2P and Freenet.