AVR is a 8- and 32-bit microcontroller core developed by Atmel. AVR was chosen as microcontroller for the first generations of the popular Arduino SBCs.
Atmel uses the AVR core in both 8- and 32-bit microcontrollers. All AVR devices use a modified Harvard architecture and a RISC core, and have a (near) 1 MIPS/MHz performance. None can execute code from external memory or RAM. The series includes the ATtiny, ATmega, ATXmega, and AVR32 controllers. The Arduino development board uses an ATmega32.
This tag should be applied to questions which involve the issues specific to AVR microcontrollers. It need not be applied to all questions which involve an AVR, for instance, questions about work within the Arduino environment should receive the arduino tag.
What does AVR stand for? According to Wikipedia:
Atmel says that the name AVR is not an acronym and does not stand for anything in particular. The creators of the AVR give no definitive answer as to what the term "AVR" stands for. However, it is commonly accepted that AVR stands for Alf (Egil Bogen) and Vegard (Wollan)'s RISC processor.