A visual mixer takes the signal from multiple cameras and allows you to control which shot is displayed to an output - like an encoder. All visual mixers will allow you at least simple transitions (like fade or wipe) between different shots. Often, they will include more advanced features like playing back previously recorded video or showing images or graphics.
Blackmagic Design have a great range of visual mixers that start from around £900. They support 4K HDMI and SDI cameras and can handle graphics, recorded assets and much more. We recommend checking the Blackmagic site - it's got loads more information!
Roland also offer great visual mixers. Some support V-link - which allows the visual mixer to be controlled by Roland electronic instruments.