I had a long IT career starting in the late 60's, but was a software engineer and never fooled around with the early microcomputer kits (Altair 8800 - 1975 , Bellmac 32 microprocessor - 1980), but now I'm retired and started fooling around with the Raspberry Pi my son gave me for Christmas.

The Raspberry Pi is a $35 single board computer the size of a deck of cards that you can use for all sort of brilliant and useful things, from learning to program, to making robots, to Tweeting when birds visit a nesting box, to running an X server.
It was developed in the UK by the Raspberry Pi Foundation with the intention of promoting the teaching of basic computer science in schools. They used Broadcoam's system on a chip (SoC) which used ARM's processor architecture.
Note: 95% of smart phones and tablets use ARM's Cortex™ processors (or licensed versions like Exynos manufactured by Samsung) running from 800MHz to 1.2 GHz as of 2013. The ARM classic processor in the Raspberry Pi runs at 700 MHz.

Model-B specs
Broadcom BCM2835  system on a chip (SoC)
700 MHz ARM1176JZF-S core
   (ARM11 family, 32-bit RISC with ARMv6 instruction set)
Videocore 4 GPU - Capable of BluRay quality playback,
                  using H.264 at 40MBits/s.
256 MB SDRAM (shared with GPU)
I/O ports:
2 USB
1 HDMI
1 Composite Video (RCA)
1 Audio
1 Ethernet 10/100
1 GPIO
1 SD Card
1 Micro USB power in

Operating Systems:
Raspbian - An operating system based on Debian
    (GNU licensed versions of Linux or BSD) optimized for the Raspberry Pi hardware. 
ARCH -  Linux port for ARM devices
RaspBMC - XBMC media center distribution for Raspberry Pi
OpenELEC - XBMC media center distribution for Raspberry Pi
RISC OS - A very fast OS
Pidora - a Fedora (Linus) Remix optimized for the Raspberry Pi

See Operating Systems

X10 Smart Home Automation:
  Raspberry Pi - X10 control
  X10 Smart House Home Automation Using Debian Linux
  DIY Your Own Home Audio and Lighting Automation with a Raspberry Pi

ARM - A RISC architecture developed by British company ARM Holdings
BCPL - Basic Combined Programming Language
CPU - Central Processing Unit
Distro - OS distributions 
GPIO: General purpose input/output
GPU - Graphics Processing Unit
NOOBS - New Out Of Box Software
   An Installer walks you through the download and installation of a specific distro.
RISC - Reduced instruction set computers
SDRAM - Synchronous dynamic random-access memory
SIMD - Single instruction, multiple data
SoC - System on a chip
See Glossary at RaspberryPi.com

Adapters:
Edimax Technology Co., Ltd EW-7811Un 802.11n Wireless Adapt