Atmel is the creator of microcontrollers like the AVR line (ATMega and ATtiny) and SAM devices. Arduino microcontrollers use Atmel's AVR line of devices.
Popular for its 8- and 32-bit avr line of atmega and attiny microcontrollers, Atmel products are widely used by hobbyists and industry professionals alike. The AVR chips are the basis of the popular arduino line of prototyping platform boards. Atmel/Microchip also offers a line of 32-bit ARM-based microcontrollers called "SMART Atmel microcontroller" or SAM. Some common Atmel/Microchip boards and platforms include AVRISP-mkII, the STK500 and STK600, AVR Dragon, and SAM [E70/V71] Xplained.
The Atmel Studio IDE (and, in previous years, AVR Studio), is based on Microsoft's Visual Studio. It is a code development, upload, and debugging tool for AVR and SAM microcontrollers. Atmel Studio can be downloaded from the Microchip website.