Wow, if you were any greener you'd have chlorophyll
Arduino is a microcontroller development board. Unlike PIC, AVR, and other microprocessors, these things have added peripherals and a 'bootloader' that allow the user to easily program it.
As far as what's it's good for? Well, the Arduino, like any other microprocessor requires a time investment to learn how to program it. Once you have the basic down you can accomplish things with a microcontroller that you would otherwise need to sit down and plan an entire circuit for. For example, a simple time delay action could be done with a dedicated circuit with a 555 timer, a couple resistors and a capacitor, or it could be done with a microcontroller.
The advantage to a microcontroller is that you can change the behaviour of your circuit simply by reprogramming it. To change the delay in the previously mentioned dedicated circuit, you would need to use more expensive parts like potentiometers to be able to adjust the timing, or you would need to swap out parts.
You should try to avoid the 'everything is a nail when I have a hammer' outlook when you have a microcontroller though. Just because a microcontroller makes it easy to build, does not mean it's the best option. They tend to be a little frail compared to some of the more dedicated components, and if the circuit ever fails it is harder to troubleshoot because the microcontroller's program is not usually made public. How do you fix something if there is a chip that you have
no clue how it actually works?
They can add value to your toys be keeping some of the more advanced circuits smaller in size, and still be able to complete more complicated tasks.