http://www.dropbox.com/File hosting services let you mirror a directory on your computer to the cloud (internet).  This is useful for backup and for sharing files across computers.
There are a wide variety of such services available.  The most popular is probably Google Drive.  Like most Google products, Google Drive is absolute junk.  It has a horrible user interface, doesn't work reliably, and I can't figure out how to use many of features.  Given how horrible it is, it is unsurprising that it is so popular with modern morons.  I have also looked at many other file hosting services and they are generally bad.  By far the best is Dropbox.  It has a clean user interface, is reliable, and has many useful feature like file versioning in the paid offering of the service.