The transmitters in the monitors work best when they are within 10 feet of the baby. This is why you should have the monitor on a nightstand near the crib, but yet you don't need to have them so close that when the baby gets older, he/she can reach out and touch it. You can carry the receiver with you or leave it plugged in by your bed during the night. The basic models have a switch you use to turn them on and off and to adjust the volume. You can also use batteries when you want to take the receiver with you, rather than having to plug it into an outlet in each room.
Some of the monitors on the market have a light display that flashes to let you know when the baby is crying. You don't need to have the volume turned up if you use this feature, but it does mean that you have to keep looking at the monitor. Top of the line monitors have a camera (a video monitor) that you can attach to your VCR. This lets you watch the baby without actually being in the same room.
There are also disadvantages to using a baby monitor. If you have a cordless phone, the baby monitor can pick up the conversation so you need to be discreet. Even talking in the same room as the baby will transfer the sound of your voice to the receiver, so you can't complain about visitors that won't go home. In once case we had neighbors with the same monitor on the same frequency and we could hear their entire night-time routine if our monitor was on. You also have to make sure that you keep the monitors away from bathtubs and pools because they are electrical devices.
Overall a baby monitor is a very useful thing to have. They give you a sense of comfort and let you get on with your day while your child is sleeping. Never forget that a monitor can NEVER replace proper parental care. Never leave your baby unattended for long periods (even while sleeping chech on them from time to time).
