A bus is defined as one where at least one of its signals is bidirectional (e.g. I2C, 3 wire (Select, Data IN/OUT, Clock) SPI) and an interface is where all signals are unidirectional (e.g. 4 wire SPI (Select, Data IN, Data OUT, Clock); however, certain devices can switch from a bus to an interface and vice versa e.g. Secure Digital (SD) card and certain SPI devices.