Hyundai Repair: Speed Sensor, vehicle speed sensor, hyundai elantra

Question
I am trying to find where my speed sensor is located for a 2001 Hyundai Elantra

Answer
Hi, Alysia.  Unfortunately, Hyundai uses the term speed sensor to refer to a number of items.  I can think of four.  If you can answer the following questions, it'll help me know to what you're referring.  Answer as many as you can, and also give me any additional information you think might be helpful.

Is this an input, output, or vehicle speed sensor?  
What does this speed sensor operate?  
What are your symptoms?