ATMega is a family in the AVR series of Atmel microcontrollers. This is the main microcontroller of the Arduino prototyping platform. Please Specify the specific part number in your question.
The ATMegas are 8-bit RISC microcontrollers using a Harvard architecture. They have 4 to 256 kB program memory. Like all Atmel AVR microcontrollers, they are supported by various (open) toolchains. The popular arduino is based on an atmel avr chip.
They are the bigger brother of the attiny microcontrollers, and have more memory and features.