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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 darknet.

When Target was hacked in 2013, customer card details turned up on darknet 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 darknet, virtually anonymous payments are easier than ever.

Just this week in Australia, a news investigation revealed that an anonymous darknet 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 darknet 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 darknet 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 darknet 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 darknet is what you'll find deep in the blackest waters below. The darknet is the 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 darknet 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 deep web and the darknet, but that level of secrecy is also sought by criminals.

This isn't just a matter of heading to "darknet.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.

Using software originally known as The Onion Router (think layers and layers of encryption), Tor secures traffic by routing it through a network of secure relays that anonymize traffic. These relays are run by volunteers around the world who donate their server bandwidth.

Think of it as a network of safe houses: You travel through underground tunnels that run along the lines of the streets above, and you pop out where you want using safe houses donated by fellow network users.

But with links on the darknet typically just alphanumeric strings of nonsense (think kwyjibo.onion) it can be very hard to know what you're getting.

It's important to remember that Tor isn't illegal software, just as torrenting software doesn't do anything illegal until you use it for sharing pirated movies. Tor says plenty of"normal people" use its service, as well as citizen journalists, whistleblowers, law enforcement agencies and, according toHuman Rights Watch, Chinese dissidents. Tor estimates that onlyabout 4 percent of trafficover its network is for hidden services (or dark web content); the rest is people accessing regular internet sites with greater anonymity.

Still, wherever you have anonymous traffic on hidden networks, the criminal activity will follow.

It's the darknet after all -- be careful what you click for.

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Darknet 101: Your guide to the badlands of the internet - CNET - CNET