I just bought my Raspberry pi and I will try to explain stuff about gpio and different usage of the raspberry pi. There might be mistake in some of my explaination and I hope there will be people to notice them.
But first, what is a raspberry pi ?
Here is the wikipedia answer:

The Raspberry Pi is a credit card sized single-board computer developed in the UK by the Raspberry Pi Foundation with the intention of stimulating the teaching of basic computer science in schools.

The Raspberry Pi has a Broadcom BCM2835 system on a chip (SoC), which includes an ARM1176JZF-S 700 MHzprocessor, VideoCore IV GPU, and 256 megabytes of RAM. It does not include a built-in hard disk or solid-state drive, but uses an SD card for booting and long-term storage. The Foundation’s goal is to offer two versions, priced at US$ 25 and US$ 35. The Foundation started accepting orders for the higher priced model on 29 February 2012.
The Foundation provides Debian and Arch Linux ARM distributions for download. Also planned are tools for supporting Python as the main programming language, with support for BBC BASIC, (As “Brandy Basic”, the BBC BASIC clone), C, and Perl.

Well, to put it simple. A raspberry pi is a little microchip (or a Plug Computer) that can runs linux and HD video. There are two model who are almost the same (model A and B, the B got a  second usb port and an ethernet controller).

Why bother ?
Because the model A costs US$ 25 and the model B costs US$ 35. It means, for thirty dollars you can have a linux server or a media center or even play with the gpio !

Aw, by the way you must be thinking. What is gpio ?
Go go wikipedia :

General Purpose Input/Output (GPIO) is a generic pin on a chip whose behavior (including whether it is an input or output pin) can be controlled (programmed) through software.GPIO pins have no special purpose defined, and go unused by default. The idea is that sometimes the system integrator building a full system that uses the chip might find it useful to have a handful of additional digital control lines, and having these available from the chip can save the hassle of having to arrange additional circuitry to provide them. For example, the Realtek ALC260 chips (audio codec) have 8 GPIO pins, which go unused by default. Some system integrators (Acer laptops) employing the ALC260 use the first GPIO (GPIO0) to turn on the amplifier used for the laptop’s internal speakers and external headphone jack.

Again, to put it simple. GPIO are pins that let you be able to plug motors, lcd, led I am a noob!

Yes, yes we’ll try to do it together !

Well I think you already understood that theses pages are by a noob for noobs, I will try to explain things as simple as possible so everyone can understand it, so feel free to give advise or ask if you didn’t understand something !