Humor is a way of presenting things that are amusing. One way to make something humorous is to present something that is absurd, illogical, or lacks common sense. Jane Austen uses many absurd characters to present humor. One is Mrs. Bennet, who is always going on about her goals to get her daughters married, who acts contrary to social requirements, and is always going on about her nerves. Another is Mr. Collins who has no mind of his own, always accepts Lady Catherine's commands, and yet feels very proud about his position as clergyman. A third is Lady Catherine herself, who is rude, condescending, and who traveled all the way to Longbourn to command Elizabeth not to marry Darcy, even though Lady Catherine herself claimed that such a union would be an impossibility.
Another way to present humor is through wit. We see a great deal of wit in Elizabeth who is very quick to point out the amusement and irony of a situation. For instance, when Lady Catherine demands to know if she is engaged to Darcy, Elizabeth quickly points out that Lady Catherine has already "declared it to be impossible" (Ch. 14, Vol. 3).
A final way in which we see Jane Austen employ humor is through irony. Austen uses many different types of irony, including situational irony, in which there are turns in the plot, and dramatic irony, in which the character's words come back to haunt them. Irony can be amusing because it is intelligent and witty.