The OLIMEXINO-NANO is a tiny board with an Atmel ATmega32U4 AVR microcontroller which is loaded with Arduino Leonardo firmware. It can be programmed via a micro-USB cable in the same way as the standard Arduino Leonardo, and features both a microSD card slot (on the bottom side) and a UEXT connector in addition to a proprietary miniature shield interface.
The miniature shield interface allows you to plug in and stack mini shields in the same way you would on a full-sized Arduino with full-sized shields. You can see compatible mini shields in the Optional Recommended Products list below.
The UEXT connector allows you to add another peripheral module easily. The UEXT can provide I2C, RS232, SPI and power to the external device. There are dozens of UEXT modules available. Please see the UEXT Boards Category for a full list.