In "The Adventures of Huckleberry Finn," how are Huck and Tom different?

Huck is much more logical and practical than Tom is.  Tom has grand visions and imaginative dreams fueled by the fact that he has read one too many adventure novels in his day.  As a result, he complicates every single issue in order to make it more adventurous and exciting.  He forms a "band of robbers" and puts these young boys through a whole list of ridiculous shenanigans, all in the name of meeting up to his high expectations of what a grand adventure should be.  He submits Jim to days of imprisonment at the end of the novel just so that he can rig a more "realistic" (which is, in reality, not more realistic) prison and escape scene.  For Tom, all else is sacrificed on the altar of a fantastic adventure.

Huck is much more realistic, and is often dragged along by the sheer force of Tom's enthusiasm for an adventure.  Huck is a smart kid who wants to go about doing things in the most efficient and logical way possible.  He sees solutions to problems, and goes about fixing them.  He is also more of a tag-along than Tom is.  Tom rustles up people and situations, taking charge and causing commotion.  Huck pretty much goes with the flow, doesn't like ruffling feathers, and is much more about keeping the peace.  As he mentions on the raft with the duke and the king,

"I learnt that the best way to get along with his kind of people is to let them have their own way."

Huck doesn't challenge the status quo; he reads people well and does what it takes to keep things smooth and happy.  For Tom, that is boring--where's the excitement in that?  Tom often chastise Huck for being no fun, and having no imagination or sense of thrill.  It just shows the differences between these two timeless characters.  I hope that helped; good luck!