What’s the difference between AVR and ARM?

Both AVR and ARM refer to families of devices. The ARM architecture was developed by the ARM company and is licensed to other companies, while the AVR architecture was developed by Atmel and pretty much stayed within Atmel (now owned by Microchip).

Both AVR and ARM are microprocessors. AVR never appears as a stand-alone microprocessor, but is always integrated with memory, input-output ports, and other peripherals to make a microcontroller. ARM, on the other hand, is available both as part of a microcontroller, as well as a stand-alone microprocessor.

The AVR based microcontroller line started with relatively simple and slow 8 bit processors, and the product line has since grown to include 16 and 32 bit processors.  Designed from the start to be the core of a microcontroller, the AVR processor has efficient commands for manipulating individual bits in input-output ports, while the more generic ARM processor might lack these features.

ARM based microcontrollers, on the other hand, are typically 32 bit devices with more complex peripherals, with substantially more memory, and running at speeds greater than AVR based devices.