There are many names, Deep web, Dark web, Deepnet, Invisible Web, or Hidden Web, and it is easy to confuse this with the encrypted web or the Tor Web. The Tor web is an entirely different conversation it is based on web sites that are open for business but are hidden from tracking the IP address back to the source, this part of the web is for those who guard their privacy for a number of reasons not all are illegal buy many are. The deep web is simply the part of the web that is not indexed by the standard search engines, like Google. It includes things like Web pages that are not linked to other pages so they are not crawled by the web spiders that index the data for the search engines, it also includes the data that is accessed via web scripts that pull data from databases. The Deep Web also contains a lot of things like embedded systems that are not intended for public access, this is a very broad category. Many old technologies for controlling our world were updated with a small CPU and a stripped down version of Linux to run a web page to access the controls and they eliminated the old dial-up line in favour of the speed and convenience of an internet connection. Many people have home routers that connect them to the internet they are out there too. The Deep web also includes what is being called the "Internet of Things" they all connect to the internet to make our life more convent. With TCP/IP v4 we actually ran out of addresses a long time ago, but we have worked around that with Network Address Translation (NAT), that is complicated and not part of this topic, what is important for us is TCP/IP v6 that is coming out now soon everything will have its very own IP Address no matter where it goes or how it is connected it will have the same address. This will make things faster and more convent.
It is tempting to think that there is security though obscurity, meaning that if you can't find it on Google that no one will find it. That cannot be farther from the truth. There are many search engines that do not do the standard crawl of web links they do IP Scans, more like a dragnet across the Internet. We all know it is important to change the passwords on our electronics, but many people don't and many devices don't even have security settings. There are thousands of IP Cameras on the Deep web that are open for viewing inducing baby monitors, traffic cameras and security cameras. There are thousands of home routers that the owner never changed the default password.
With TCP/IP v6 and the Internet of Things there will soon be many more things out there in the Deep Web. How about a Crock-pot. This is a very convenient device, do you even care if it is secure? What if you don't reset the default password, can some stranger turn off your dinner? or more scary bypass the thermostat and turn it on high? How about that fitness tracker you wear on your wrist that tracks your location and has a microphone to use with your cell phone, many do not have any security at all. Light bulbs, washing machines, thermostats, fridges and locks are all on the internet today and the pace of new things is increasing. It is a given that nothing is 100% secure, but how much of your security are you willing to give up for convenience?