The Arduino UNO is a fairly small development micro controller with lots and lots of accessories as well as ready made so call shields.
A shield is basicly a board that you plug right on top of the Arduino which gives you a specific set of functionality. There are a lot of these shields that do different things. For instance there are shields that gives you WIFI, others that give you a wired connection, motor drivers etc. These are usually a simple way to get extra functionality quickly at an increased price and size.
Boards and links
There are a number of different Arduino boards. They are go from small to tiny and most of them are fairly cheap. A very good place to start out looking at arduinos is their official site at Arudinos official homepage. When it comes to getting cool arduino related stuff is at Sparkfun. So far I have not bought anything from them thought, but I expect that to be only a matter of time before I do.
The standard Arduino UNO board is small. Only 68.6 x 53.4 mm in size. There are other boards that are even smaller. The Arduino Pro mini for instance is only 33 x 18mm and does not come with any type of USB port that allows you to connect it to your computer directly. In this case you will need a USB to serial interface to program the board. The Arduino UNO however comes with a full size USB Type B connector.
The UNO gives you 16 digital pins as well as 6 analog input/output pins. These are generally called GPIOs or General Purpose Input/Output.
The digital pins can be set to either high or low (1 or 0) and can both send and recieve data. These can be used to turn on and off LEDs as well as recieve on or off sigals from an outside source (buttons, sensors etc).
The ananlog pins vary their voltage which allows you to get a varied input. This is used in applications where there is more than just two values, for instance when measuring temperature and humidity.
A good thing to have when doing Arduino stuff is a USB to serial interface. It is basically a USB connector with 5-6 pins. The general pins that you find on them all are 5/3.3V, Ground (GND), RX (Recieve), TX (Transmit).
The only serial interfaces that are available here in Sweden where I live works at 5V, but there are versions that allows you to switch manually between 5 and 3.3V. I would recommend getting one that allows you to switch voltages. As I will be bringing up later on in this blog it is also possible to use the Arduino UNO as a FTDI adapter (as in using the UNO instead of a dedicated adapter).