The body of sin is our flesh (not literal skin).
It was crucified with Christ.
We are now free to marry Christ because our husband is dead. This looses us from the law... The power of sin is in the law.
I'm not going into details because I will get ridiculed out of this thread in no time flat. The questions you asked are directly related to Romans 7. I hope this helps.
Rom 7:1 Know ye not, brethren, (for I speak to them that know the law,) how that the law hath dominion over a man as long as he liveth?
Rom 7:2 For the woman which hath an husband is bound by the law to her husband so long as he liveth; but if the husband be dead, she is loosed from the law of her husband.
Rom 7:3 So then if, while her husband liveth, she be married to another man, she shall be called an adulteress: but if her husband be dead, she is free from that law; so that she is no adulteress, though she be married to another man.
Rom 7:4 Wherefore, my brethren, ye also are become dead to the law by the body of Christ; that ye should be married to another, even to him who is raised from the dead, that we should bring forth fruit unto God.