They're called lots of names, such as external power supplies, power blocks, power bricks, and chargers, and they're an integral part of a wide variety of electronic products.
Power adapters convert AC voltage from a wall outlet to the DC voltage that charges the batteries in - or directly powers - cell phones, cordless phones, laptop computers, calculators, tools, digital cameras, camcorders, answering machines, and many other devices.
Many power adapters are very energy efficient. Some of them still consume power when they're plugged into a wall outlet but aren't ever connected to their compatible devices.
Newer power adapters generally shut themselves off when the batteries in the device to which they are connected reach a full charge. In addition, they consumed no power when they were left plugged into an outlet but weren't connected to a device.
Older power adapters often don't shut themselves off. They continue to consume power when batteries reach full charge - even when the adapters are plugged into an outlet but no longer are attached to a device.
There's no sure way to tell by looking at a power adapter whether or not it's an energy waster. However, it may hum or feel warm to the touch when it's using power.
So, what can you do?
First, monitor the charging process of any device and unplug the power adapter as soon as the batteries are fully charged.
Second, unplug any power adapter that's not connected to a device or one that powers a device you're not actively using, except those that have clocks or would lose their settings if they were not powered.
Finally, when you go shopping, look for products now coming on the market with power adapters that are "Energy Star" qualified. On average, such power adapters will be 35-percent more efficient than conventional models and are often physically smaller and lighter.