7-segment displays are 7+1 LEDs in the same package shaped to form the number 8. From these 7 segments, any other number can be formed, some letters also.
Each segment is allocated a letter to identify it, a to g.
Displays come in 2 variations, Common Anode or Common Cathode - these are shown right.
In most circumstances (unless specifically stated by an IC, an LED must have a resistor in series with it to prevent damaging it.
For voltages between 6 to 12v, a 470 ohm resistor will be fine but to get maximum brightness, without damaging it, you should work out the correct value.
The LED page has more information on how to do this.
You can make larger displays by combining them - to find out more go to
Building Giant 7-Segment Displays.