The iRiver AFT 100 Mobile FM Transmitter is a device to be plugged into the cigarette lighter of your car and the other end into any audio device with headphone output (1/8" audio jack like on a MP3 player, CD player, or laptop). It then broadcasts a radio transmission that you can tune into with your car radio.
The iRiver AFT 100 allows you to change the station it broadcasts too -- any FM station is valid. Because it's not a very powerful transmitter, you will frequently run into interference from other stations. You can preset 3 stations or manually adjust the station it broadcasts to. You will need to take some time to discover what stations are used in the areas you frequently drive in. It's very much a trial-and-error process. If you've used satellite radio systems, the transmitters on those are much stronger, so beware.
It functions quite well, once you find the right stations. The backlight on the device is blue and easy to see, even from several feet away. It's also easy to change the station by feel alone, so you don't have to take your eyes off the road. It also saves the last station you were tuned into, regardless of if it's a preset or not. If your cigarette lighter is placed in certain areas you may bump the iRiver with your knee or leg, as it protrudes about 3-4 inches and has a wide head for the large display.
Keep in mind that any time you have two distinct volume controls you need to adjust them accordingly. For example, don't turn the volume on your MP3 or CD player up all the way as the sound quality will go way down, regardless of how you set the volume on your car radio. On the other hand, don't set it too low, because you'll need to turn your car radio way up, and interference and static will be very audible and annoying. Play around with the settings until you discover something you like
Overall it's a good product and a cheap solution for being able to play CDs or use your MP3 player in your car if it's not built in or you don't want to pay a lot to have it added.