What are these receiver jacks called?

http://imgur.com/llc5sgm

I apologize if this question is too basic. Since I don't know what it's called, I couldn't search posts/google myself. I am trying to insert loudspeaker cable to these jacks, but those copper cable strands are too thick, so I'm having hard time inserting. I was thinking maybe there is a plug (kinda like banana plugs) that I can use to easily insert my cables to these jacks.

Can someone tell me what these jacks are called OR what kinds of plug I can purchase so I can insert my cables all the way?

Thank you!