MicroSD Shield and SD Breakout Hookup Guide
The features of the microSD Shield are as follows:
A large protoyping area where you can solder other parts for your project.
A hex converter IC (which acts as a voltage level shifter between the Arduino's 5V logic and the microSD card's 3.3V-tolerant pins). The datatsheet for that IC can be found here.
A microSD socket.
A power LED. Lights up as soon as the Arduino is powered.
A reset button connected to the Arduino's reset line.
SCL and SDA jumpers that tie the SDA and SCL pins (broken out on the R3 shield footprint) to the A4 and A5 pins to provide backward compatibility for older UNO models. These jumpers can be opened to free up pins A4 and A5 when using boards with a different I2C pin assignment like the Leonardo, Mega, and Due.