Some audio/video monitors filter out "normal" sounds and motions. The receiver is supposed to turn on only when your baby makes an unusual motion or sound, such as crying or rolling over when he's waking up from a nap. This feature is designed to extend battery life, although the receiver should be docked for overnight monitoring to keep the battery charged. We haven't tested any models with this feature, so we don't know how well it works.
Watching your child from moment to moment is far more important than going over footage from previous nights, so baby monitors don't usually make a big deal about saving video for later, whether using built-in storage or through a cloud service. They can take snapshots and short clips when they detect movement, but they won't offer time-lapse videos of entire nights at once, or let you page through hours or days of footage. Those features are useful for identifying burglars, but they don't really help you watch your child unless you're in a Paranormal Activity sequel.
Gone are the days of silently peeking into the nursery to check on your napping baby and then, whoops, accidentally waking them up (“No, no, please no!”). A baby monitor uses a camera to watch over the crib, while you carry a handheld device that lets you know what’s going on with your child at any given moment, no matter where you are in your home. There are also other monitors that track sleep or breathing either with a clip or sock.