Intro: 7 Segment Display
7 Segment Displays are going to be used anywhere you need a visual display of digits. A Timer, countdown, clock, etc are all examples of where someone might used 7 Segment Display. They are very useful, and once you understand how they work they are simple to implement.
There are multiple ways to implement a 7 Segment Display, but what I believe is the easiest and most common is to use a decoder or driver. The Drier allows you to input a 4 bit binary number and it automatically decodes it and activates the necessary segments to display the number in base 10.
The driver needs a few connections. First the power and ground are connected. Then there are 7 connections that need to be made from the driver to the LED display through a resistor. The resistor is just a current limiting component to keep from blowing the LED. Then there are 4 connections to be made to sent the binary number.
allaboutcircuits.com has a nice circuit diagram to show you an example of using 4 switches to control the 7 segment display and all of the connections that need to be made.
thelearningpit.com also has a few nice illustrations of where the LED is actually contained in the display, and how the connections are run. If you are familiar with LED's these illustrations will help you understand how simple this device is. You can also see the Truth Table for the display.
7 segments are great to use with projects and hopefully now you can see how easy they are to implement. The wires may get messy on a breadboard, but if used on a PCB all you need is the driver and a few resistors to get it going.
You can also use an Arduino to replace the driver. They have a 7 Segment Library which can be very handy if you're already using arduino in your project.