Assistive devices are anything that helps you to hear better, that aren't a hearing aid.   Assistive listening devices can be stand-alone devices that connect students to teachers, such as FM transmitters and receivers.  

An FM system picks up sound through a microphone, and transmits the sound to a receiver.  The sound then can be listened to through headphones, an ear bud, an ear mold, or be transmitted directly into a  person's hearing aid.  These systems are perfect in classroom settings.  

Another example of an assistive device are bluetooth enabled gateways such as Oticon's Streamer and Connectline Products and  Starkey's Surflink Mobile Products.  These devices allow you to pair your hearing instruments with your television, telephone, and cell phone.  

Another assistive listening technology that is becoming increasingly popular are listening (or induction) loops.  These systems broadcast sound picked up by a microphone to be broadcast to anyone in the room wearing hearing aid with a telecoil inside.  As long as the person is inside the loop, they can easily hear the signal sent through a microphone or sound system.  They are perfect for theatres, worship services, and other auditoriums.  Although not common in our community, they are something we can advocate for.  

For more information, please read this great article from the National Institutes for Health.

You can also read this article from the FDA.