From Steve Ciarcia's Build Your Own Z80 Computer:
Memory for the Z80 is organized into 8-bit quantities called bytes. Each program byte is stored in a unique memory position and is referenced by a 16-bit binary address.
Total direct addressing capability is 65,536 bytes (64K) of memory, which may be any combination of ROM (read-only memory), EPROM (erasable-programmable read-only memory), or programmable memory. Data is stored in little endian format:
[ D7 | D6 | D5 | D4 | D3 | D2 | D1 | D0 ] MSB LSB (Most significant bit) (Least significant bit)