What does the 'A' trailing letter in Atmel's ATtiny devices mean?
A-grade AVRs (and as of November 2014, B-grade AVRs) are minor improvements over the previous iteration; these improvements vary from chip to chip, e.g. the difference between ATtiny2313 and ATtiny2313A is different from the difference between ATmega128 and ATmega128A. Atmel has application notes that explain the difference.

AVR531: "Migrating from ATtiny261/461/861 to ATtiny261A/461A/861A"