"Arduino" as a term reflects a platform of integrated hardware and correspondend software development kits.
If you came here looking for project inspiration, please see the microcontrollers page.
From a hardware perspective, Arduino offers a range of electronic boards, which can be used for the easy setup and control of circuits. Arduino boards are basically boards with different kinds of Atmel AVR Microcontrollers and some attached electronics (such as USB Serial Converters, power supply units and pins to access the GPIO Pins of the Atmel AVR).
From a software perspective, Arduino offers multiple tools and a software development kit.
In case you want to develop your own software for an Arduino, you can use most of the tools mentioned on the Microcontroller page.
We are also documenting problems (and solutions!) on an "Arduino Debugging" page.
If you are looking for information about certain functions or variables, the Arduino Reference is a great place to start.
You can power Arduino boards in various ways, many of which are summarized here.