Just for the record, Extender is a Marketing term and has nothing to do with Real technology.
The real term is
Repeater. Meaning, the second unit connects to the source Router via Wireless and then further transmit the signal.
Because the Repeater unit do both the Receiving and Transmitting, Repeaters cut the Bandwidth into half of what it is in the point of their Installation.
E.g., if a Wireless source gives 100 Mb/sec. a foot away from it and at 50 feet away there is a Repeater that gets 40Mb/sec. the signal past the Repeater would be only 20Mb/sec. Then if you have a Wireless device 20 feet further from the Repeater it would get only 10 Mb/sec. (or less).
The numbers above are for illustration only, they can vary up and down according to the environment and the hardware.
Using a unit that connects to the source via CAT cable is known as using and additional Access Point, technology wise it is much superior to the use of a Repeater.
Example - Using Wireless Routers (or Modem/Wireless Router) as a Switch with an Access Point -
http://www.ezlan.net/router_AP.html