The Olimexino-32U4 is based on the Arduino Leonardo, featuring an AVR ATmega32U4 microcontroller. The ATmega is pre-programmed with the Arduino bootloader that allows you to upload code to it via USB using the open-source Arduino development environment.
The ATmega32U4 has built-in USB communication, eliminating the need for a secondary processor. This allows the board to appear to a connected computer as a mouse and keyboard in addition to a virtual (CDC) serial / COM port. It also has other implications for the behavior of the board — particularly reset behavior compared to earlier Arduino boards. It also means that there are now separate Serial and Serial1 classes for the virtual serial driver and the hardware serial port, respectively. This also has an effect on the capabilities and assignments of some pins, e.g. SPI and TWI.
The ATmega32U4 allows for serial (CDC) communication over USB and appears as a virtual com port to software on the computer. The chip also acts as a full-speed USB 2.0 device, using standard USB COM drivers. It requires no drivers to be installed for Linux or Mac (for Windows a .inf file is included with the Arduino IDE software).
The main differences between this board and the official Leonardo are a mini-USB connector instead of micro-USB, a battery connector in addition to the power jack, a UEXT connector for additional peripherals, a 3.3V/5V selection jumper, and through-hole header pads at 0.1" pin spacing in addition to female headers at the traditional Arduino offset. The Arduino Shield connectors are a hybrid of R2 and R3 configurations. They have the added SDA and SCL pins of R3 on one side, but do not have R3's IOREF and unused pin on the other. The power selection jumper duplicates the job of the IOREF pin, only it is a manual setting rather than automatic.