There are two basic types: audio and video/audio. Some are analog, others are digital. All monitors operate within a selected radio frequency band to send sound from a baby's room to a receiver in another room. Each monitor consists of a transmitter (the child/nursery unit) and one or more receivers. Prices range from about $25 to $150 for audio monitors and about $80 to $300 for audio/video monitors.
Type: After considering the options, weighing the relative advantages, and experiencing many firsthand, we determined our ideal monitor would be an RF (radio frequency) video monitor rather than one of the two main alternatives: a Wi-Fi (or cloud-based) model that you can check on your phone, and bare-bones audio-only speakers. (We approached our research with an open mind and gave an equal chance to all three types.) Since the best audio monitors cost far less, we have recommendations for both video and audio types—and we answered the question, What about a Wi-Fi baby monitor? with a firm conclusion that RF video can better provide what most people want: a clear view of a baby, a secure connection, and a dedicated monitor that can operate in the background without tying up your phone.
It really depends on what you feel most comfortable with. There are audio monitors that allow you to listen to any noise coming from the nursery, vital monitors that track sleep and breathing and video monitors that add sight to sound. Babylist parents overwhelmingly choose video monitors. The security of seeing what your child is up to—like if they’ve gotten tangled in their swaddle, pulled their diaper off or climbed out of the crib—can be worth the extra cost of a video monitor.