We get a fair number of questions about components in the Etray, this is a quick article to help clarify what certain parts are:
The Beaglebone Black (BBB) is where the SD card is located. It handles the printer to computer interface. When you have Octoprint (your machine control interface) open in your brower, that's there thanks the the BBB.
The RUMBA is the motherboard of the printer. The stepper drivers that run the motors (X, Y1, Y2, Z, and E0) are all plugged into the RUMBA.
The Etray PCB is where the ribbon cables are connected. It helps run electrical signals from the RUMBA up to the tool head. There is also a gantry PCB next to the X stepper motors and the toolhead PCB on the toolhead. They are all connected by the Ribbon cables.
The Etray fan is the 80mm 12V fan in your etray. The shroud covering it is to help direct the air towards to electronics to cool them off. Particularly, the stepper drivers can get fairly hot.