Dramatic irony is created when the audience or readers know more than the characters do.  Mrs. Bennet, often, does not understand that her behavior is completely inappropriate or that she is an object of ridicule and disdain; these moments are examples of dramatic irony.  For example, when Mr. Darcy suggests that the company the Bennets keep in the country is not as varied as what one finds in London, Elizabeth insists that people have a tendency to change so much that there is always something new to see in them. Moreover, Mrs. Bennet responds in such a way as to indicate that she was "offended by his manner of mentioning a country neighborhood."  After her outburst, Mr. Darcy silently turns away.  Mrs. Bennet "fancied she had gained a complete victory over him [and] continued her triumph," then went on to say that the Bennets dined with twenty-four local families, an apparently abysmally small number as "nothing but concern for Elizabeth could enable Bingley to keep his countenance."  His sister, however, is less discrete.  Mrs. Bennet does not comprehend her own rudeness or the fact that she has made herself and her family appear provincial in front of their guests.  She thinks she is "victorious" when she is really only ridiculous.

Situational irony is created when what we expect to happen differs from what actually happens.  For example, when Mrs. Bennet is discussing the Lucas girls with Mr. Bingley, she says, 

"It is a pity they are not handsome!  Not that I think Charlotte is so very plain—but then she is our particular friend."

Then, in the next moment, when Mr. Bingley compliments Charlotte Lucas's personality, Mrs. Bennet is quick to say, 

"Oh! dear, yes [she is pleasant]; — but you must own she is very plain."

Now, given Mrs. Bennet's defense of Charlotte's looks, as well as her pointed proclamation that Charlotte is such a good friend of the family's, one would not expect—in her very next speech—to hear her insult Charlotte; therefore, when she does, the reality differs from expectation and situational irony is created.  We can assume she only insults Charlotte because she wishes to contrast Charlotte's looks with her own daughter's (Jane) beauty in the next line.  It makes Mrs. Bennet look quite insincere.

While dramatic irony is a classification of situational irony, along with tragic irony, and Socratic irony, the difference is that in dramatic irony the words of characters come back to haunt them, whereas situational irony involves a turn of events in the plot.In Pride and Prejudice, an example of dramatic irony can be seen when Elizabeth begins to reproach herself after reading Darcy's letter. When Darcy proposes to her, she lays out many accusations about his treatment of her sister and his treatment of Wickham , concluding that he is a prideful and detestable man. After reading his letter, she is forced to eat her words and realizes that all of her accusations stemmed from prejudicial judgement of his...