Summary
Because the Yún has two processors, the summary section shows the characteristics of each one in two separate tables.
AVR Arduino microcontroller
Microcontroller    ATmega32u4
Operating Voltage    5V
Input Voltage    5V
Digital I/O Pins    20
PWM Channels    7
Analog Input Channels    12
DC Current per I/O Pin    40 mA
DC Current for 3.3V Pin    50 mA
Flash Memory    32 KB (of which 4 KB used by bootloader)
SRAM    2.5 KB
EEPROM    1 KB
Clock Speed    16 MHz
Linux microprocessor
Processor    Atheros AR9331
Architecture    MIPS @400MHz
Operating Voltage    3.3V
Ethernet    IEEE 802.3 10/100Mbit/s
WiFi    IEEE 802.11b/g/n
USB Type-A    2.0 Host/Device
Card Reader    Micro-SD only
RAM    64 MB DDR2
Flash Memory    16 MB
PoE compatible 802.3af card support (see the note below)