The ATmega32A is a functionally identical, drop-in replacement for the ATmega32. All devices are subject to the same qualification process and same set of production tests, but as the manufacturing process is not the same some electrical characteristics differ.
The ATmega32A is a low power version (Atmel calls it picoPower) of the ATmega32.
XMEGA is a new family of microcontrollers from Atmel. XMEGA devices have:
2nd generation of picoPower technology
Innovative Event System for fast, CPU-independent inter-peripheral communication
4-channel DMA Controller boosts MCU performance
100% predictable timing
Fast 12-bit ADC and DAC
Fast Cryptography support for AES and DES