Dark Web, TORTOR stands for The Onion Router due to it's multiple layers.
TOR was developed by the US Government under DARPA to provide itself and at-risk social activists from their own authoritarian governments. It is now an open source project no longer under government control.
All web sites on the TOR network are also known as the "dark web" or "deep web" because they cannot be accessed via the normal internet web browser. There is no DNS system as is used with typical web traffic.
The TOR network is run on over 6,000 exit nodes with a far larger number of relay nodes operational at any given time. The actual number varies but is growing all the time. Anyone can apply to provide a node which will be available to the network whenever your computer is online. Nodes come and go.
TOR provides anonymity. VPN provides encryption. Use them together for the best of both worlds. You can surf any sites on TOR you can over the Clearnet unless the site publisher has placed restrictions on access via TOR. If you jsut connect to TOR your ISP can tell only that. If you use a VPN your ISP cannot even tell your also on TOR network.
TOR can often be much slower than your normal ISP internet connection both because of the numerous hops from node to node as well as the fact that individual nodes may be on smaller individual computers along the way. your connection will be only as fast as the slowest node between you and your destination.
Data between you and each node is encrypted so no one along the way can see your data.
Here are some relevant links.