A dongle plugs into the USB port on a laptop or desktop PC to provide internet over mobile broadband. Generally, they only provide internet to the device they are plugged into and are small, discreet and require no installation.
Mobile Wi-Fi (or MiFi) units are battery-powered devices that can share a single mobile connection with multiple devices over Wi-Fi.  That means you can use them wirelessly and connect almost any internet enabled device. It also means you can share a single 3G, 4G or 5G connection with multiple Wi-Fi devices at the same time, including your tablet, desktop PC or games console.