When data is sent from the controller to a peripheral, it's sent on a data line called COPI, for 'Controller Out / Peripheral In'. There is always only one controller (which is almost always your microcontroller), but there can be multiple peripherals (more on this in a bit). The side that generates the clock is called the 'controller', and the other side is called the 'peripheral'. In SPI, only one side generates the clock signal (usually called CLK or SCK for Serial ClocK).
You might be thinking to yourself, self, that sounds great for one-way communications, but how do you send data back in the opposite direction? Here's where things get slightly more complicated. You can also see OSHWA's resolution here. Check out this page for more on our reasoning behind this change. SparkFun has joined with other members of OSHWA in a resolution to move away from using 'Master' and 'Slave' to describe signals between the controller and the peripheral. Note: You may not recognize the COPI/CIPO labels for SPI pins.
