"But whoever causes one of these little ones who believe in Me to stumble, it would be better for him if a millstone were hung around his neck, and he were thrown into the sea. If your hand causes you to sin, cut it off. It is better for you to enter into life maimed rather than having two hands, to go to hell, into the fire that shall never be quenched--where 'their worm does not die, and the fire is not quenched.' And if your foot causes you to sin, cut it off. It is better for you to enter life lame, rather than having two feet, to be cast into hell, into the fire that shall never be quenched--where 'their worm does not die, and the fire is not quenched.' And if your eye causes you to sin, pluck it out. It is better for you to enter the kingdom of God with one eye, rather than having two eyes, to be cast into hell fire--where 'their worm does not die, and the fire is not quenched.'"
In Mark 9:42 we find the converse to 9:37. Jesus says that it would be better for one to drown by being tied to a millstone (i.e., a large stone usually moved by a strong animal) and thrown into the sea than it would be to lead others astray by one's words, actions, or influence. This shows just how terrible of a sin it is to cause any of God's children to stumble (cf. Luke 17:1,2). It is one thing to ignore one's own duties to the Lord and end up going to hell, but it is quite another to be a stumbling block to someone else and pull them down to destruction with you!
It should be observed that Jesus here uses a different Greek word for "little ones" than He did earlier when referring to "little children." This indicates that Jesus is no longer talking about literal children but about new converts (i.e., beginners in the faith; cf. I Pet. 2:2).
"Woe to the world because of offenses! For offenses must come, but woe to that man by whom the offense comes!" (Matt. 18:7). Jesus now speaks in general about offenses and states that they "must come." This is not because it is God's will that they should come, but because the wickedness of men makes them inevitable. One would be foolish to look for a day on earth in which there will be no offenses; rather, he should make sure that he is not the cause of any of them.
Jesus uses exaggeration in Mark 9:43,45,47 to emphasize His point; that is, it is far better to undergo any form of self-denial and earthly suffering and enter into heaven (i.e., "life") than it is to be cast into the eternal fire of hell (even if the agony of self-denial we endure is comparable to the pain of losing a limb or an eye).
Let it be understood that Jesus does not desire one to literally cut off or pluck out various body parts. God's will is that we take care of the physical bodies we have been given (cf. I Cor. 6:19,20). These destructive acts, if carried out literally, would not stop one from sinning anyway since the root of all problems with sin is not in the hand, foot, or eye but in the heart (cf. Mark 7:21-23). It is likely that Jesus mentions the hand, foot, and eye because they are among the most valuable of the visible members of the body.
In Mark 9:44 we have a quotation from Isaiah 66:24. The imagery borrowed from Isaiah is that of worms feeding upon the carcasses of men. It seems unlikely that the fire and worm should be understood literally since fire is not a natural environment for worms (though God could certainly create such if He wanted to). It seems more likely that Jesus is depicting hell as a state of decay that is never complete (i.e., "the worm does not die") and of burning which does not consume (i.e., "the fire is not quenched"). Some believe the worm is a symbol of the gnawing of remorse and the fire is a symbol of actual punishment. This verse is repeated verbatim in Mark 9:46,48.