A baby monitor is essentially a radio transmitter on a fixed frequency that puts out a nice FM signal so long as it is on. Most baby monitors transmit on a fixed frequency of 49.300, 49.830, 49.845, 49.860, 49.875, or 49.890 MHz (some have an A/B switch that lets you pick one of two of these). If the monitor is in an upstairs bedroom, in a house not covered with aluminum siding, even the simplest radio scanner can easily hear all sounds from that room. The receiver in a radio scanner is typically quite a bit more sensitive than the receiver portion of your baby monitor, and the scanner can pick up the transmission from perhaps half a mile away.
Radio scanners are relatively easy to find at consumer electronics stores. Many if not all the current models include the 49-50 MHz range. The frequencies used are just above the frequencies used by the conventional analog cordless telephones sold in the '80s and '90s, and just below the frequency that VHF TV channel 2 uses.
To avoid eavesdropping, simply turn the monitor off unless it is monitoring your baby.
Because a baby monitor is really a mini radio station, there are many ways to use it once your baby grows up. A handy use for a baby monitor is to place it by a TV set speaker when you want to hear a TV show but have to be in another part of the house or out in the yard. Or you might use a monitor to help you hear a distant doorbell or beeper.
These links will help you learn more: