There are three basic kinds of monitor: audio, video, and movement or breathing.
The audio monitor is the most popular kind, and at its most basic consists of a transmitter that you place in your baby's nursery, and a receiver that lets you 'listen in', alerting you if your baby starts to cry or otherwise shows signs of waking. The very cheapest modules will have a fixed receiver that needs to be plugged into a wall socket for power, but most models nowadays have a rechargable mobile receiver which you can carry from room to room, or even outside so long as you stay in range.
The range of your monitor system will depend on a number of factors, including the construction of your home - thicker walls will reduce the range - and any sources of interference such as neighbouring monitor systems, computer wireless networks, and even microwave ovens with some models.
Interference is less of a problem these days, as most models will now let you select from a range of channels to operate on, so if you do encounter interference problems, you can switch to a new channel to hopefully clear things up. Some units only feature two or four channels which you have to manually select, while the more advanced designs now have hundreds of channels which will be selected automatically to minimize interference in the current conditions.
The more modern monitors, like so much else these days, use digital technology. What this means in practice is a clearer signal, and an increased range of operation.
Those are the basics of audio monitors, but there are other features available depending on the model. A popular one is an intercom or talkback system where you can talk to your baby through the monitor, helping you to soothe her back to sleep without the disturbance of going into the nursery. Other models may have nightlights or lullabies which you can set to play either automatically or by hand, again without entering the room, while another common feature is temperature alerts where you can see how warm the nursery is, and be alerted if it falls outside a safe range.
For most people, an audio monitor is sufficient, but some people value the extra security of having a video monitor where you can actually see your baby on a TV screen built into the receiver. If you decide to get a video monitor, make sure it has night vision capability, or it won't be much use when it's dark!
For complete reassurance that your baby is slepping safely, you can also get a breathing and movement monitor. This consists of a pressure pad placed under the cot mattress, which will sense if your baby stops breathing or shows other signs of distress, and alert you immediately.
The final point to consider when choosing a monitor system is price. The range is enormous, from only a few dollars up to a few hundred. For something as essential as a monitor though, it's worth getting the best model you can afford, as after all, if the system doesn't work well and you can't trust it, then it's effectively a waste of time.
