Arduino is an open-source, programmable microcontroller and software based on the ATMega chip. Although the Arduino is designed as a prototyping platform, it can be used in various electronics projects whether temporary or embedded. The Arduino board can be programmed using the Arduino software. The syntax for this is similar to C/C++ and Java. It is designed to be simple and easy to use, and can be operated by anyone, from beginners to experts alike.
As Arduino is an open source platform, you can get hold of the source code and schematics for it. This means you can delve as far into it as you want, even creating your own Arduino boards. There is also a large community behind it, and you can find many tutorials and projects from all over the world online.