Modems are boxes that connect your home to the Internet and World Wide Web. The main job is to translate the incoming information to a digital signal Ethernet or Fiber devices can understand. Your devices and the Internet cannot actually communicate without a handoff between them. For DSL and Cable Internet, you will have a modem; Fiber systems will generally have an Optical Network Terminal (ONT).
Technically, an ONT isn’t a Modem as it doesn’t modulate/demodulate. When speaking of fiber-optic Internet, the result for your home network is essentially the same — read on.