With these questions you are jumping into the deep end of the pool. It will take a lot background knowledge to understand the answers. For a good introduction to the topic, I recommend Nielsen and Chuang's book Quantum Computation and Quantum Information. It's a challenge, but you can get through it if you are competent at linear algebra.
Thanks for pointing me towards the book. I am reading another book on entanglement and it used dense coding as an example, but it was very brief; I wanted to get a better understanding of it. I haven't taken linear algebra yet, so I will refer back to this when I get there.