Features: 
SD card interface works with FAT16 or FAT32 formatted cards. 
3.3v level shifter circuitry prevents damage to your SD card 
Real time clock (RTC) keeps the time going even when Arduino is unplugged.  
Battery backup lasts for years 
Prototyping area for soldering connectors, circuitry or sensors. 
Works with Arduino UNO, Leonardo.,...