What is this that hurts behind my knee?

Alright I'll try to explain as best I can of where this pain is. First off, it hurts when I do leg extensions for my hamstrings. Now to find where I'm talking about stand up and put your leg on a chair and create a 90 degree angle with your upper and lower leg. Then on the inside of your leg right where you bend it to create the 90 degree angle you will feel something like a long, skinny stick. Can you tell me what this is and if you can give sources?