A desktop PC houses all its components within its case, or tower, including an independent CPU, GPU, RAM, motherboard, fans/cooling, disc drive(s) and PSU. Users typically place it either on top of or underneath their workspaces.
The monitor, or display, is separate from the tower and can be connected to it via HDMI and USB-C (for newer desktops), and VGA and DisplayPort (for older desktops). You may need to purchase your PC peripherals including the mouse, keyboard, external speakers, and monitors separately.
