Peer-to-Peer file sharing of computer files using a networking technology. P2P is a special kind of network type ('Network topology' is the fancy
description). It describes computers that are networked to each other
without a central server being in-between. It allowas users to acces media files such as books, musc, movies, and games using P2P.
The earliest one (Napster) to the latest one (BitTorrent protocol).
Several factor contributed to the widespread adoption of P2P file sharing:
- Increasing Internet bandwidth.
- The widespread digitization of physical media.
- The increasing capabilities of personal computers.
Software has been developed that can even put together a file from bits
of the file stored across several 'peers' on the network. For example
Bit Torrent can do this. The advantage is that no single computer
needs to be over-loaded when a very popular / very large file is
requested.
An overhead projector works on the same principle as a 35mm slide projector, in which a focusing lens projects light from an illuminated slide onto a projection screen where a real image is formed. However some differences are necessitated by the much larger size of the transparencies
used (generally the size of a printed page), and the requirement that
the transparency be placed face up (and readable to the presenter). For
the latter purpose, the projector includes a mirror just before or after
the focusing lens to fold the optical system toward the horizontal.
That mirror also accomplishes a reversal of the image in order that the
image projected onto the screen corresponds to that of the slide as seen
by the presenter looking down at it, rather than a mirror image thereof. 
