There are many articles, Blogs and books related to VPNs: from the very technical and complicated ones, to blank and purely theorizing, to businesses oriented.
VPNs or similar technologies allow whoever has proper credentials to connect remotely to your network in a secure way. This obviously allows remote users to access information and utilize programs without the need to be physically present in your premises. There are no direct copper or fibre optics cables interconnecting offices or individual computers, rather the Internet is the conduit: by compressing and encrypting the information traveling from the office to the remote worker, so that prying eyes can’t do much to steal and much less interpret what is being sent over “open wires”.
There are also new technologies taking place (MS’s Direct Access) that will replace the need to install more devices to our wiring closets or packages on our over-loaded servers. These are already being implemented today, and will surely make everybody’s life easier.
A complete and very good –although technical- explanation of VPNs can be found here: http://en.wikipedia.org/wiki/VPN