What is Arduino

Arduino is a tool for making computers that can sense and control more of the physical world than your desktop computer. It's an open-source physical computing platform based on a simple microcontroller board, and a development environment for writing software for the board (as said by the developers of Arduino).

Cheapest arduino choices 

In all things combined it's the next best thing since the computers were invented. Anything you can imagine you can develop with it, and if you want to go to masses you can always strip it and use the Atmega microcontroller. 

Okay there are a few defects, mainly no integrated graphics department. But there are workarounds for that, although not as powerful that there should be but still it's a possibility.

In mi opinion why it's the next best thing is because it's popularity has skyrocketed. Because of that there are endless add-ons for it. That's why I said you can do everything with it, and so much has already been done by the community. Also the community who can argue with that. If you could ever get in some kind of trouble, there is always someone who can help you.