Dialogue. The conversations between characters in a story. Without effective and true to life dialogue, a story or book is likely to fall flat. Dialogue needs to fulfill a specific purpose—it should not be random exchanges between the characters. It should provide important background information, advance the plot, or develop the characterization. A character's dialogue can also tell you a lot about where he/she is from, level of education, core beliefs, political leanings, etc.
Well-written dialogue will also distinguish one character from another. Each should have a particular speech pattern, use the same slang or cliches repeatedly, as well as any other speech characteristics that distinguish him from any other character. With very well-written dialogue, you should be able to tell who is speaking without any identification. Many beginning writers will include too much description and background information through narration, rather than letting the information flow from the dialogue. Another common problem is trying to find substitutes for “he said” or “she said.” In most cases it is best to stick with those, rather than saying “he moaned,” or “she giggled.” It is almost impossible to moan or giggle a sentence. And, in cases where it is obvious who is speaking, you don't need to identify the speaker at all. Some beginning writers also decide dialogue in a novel should be the same as regular conversation—including all the oohs, aahs, sidetracks, and repetition. The problem is if you recorded (or wrote down conversation as you hear it), you would usually find it incredibly boring and difficult to read. Good dialogue picks out a lot of the nuances of real conversation, but condenses it to its essence.
A lot of readers will judge how interesting a book is going to be by picking it up, opening it at random, and seeing how much dialogue there is. If they see long stretch of narration, they may put the book down and select another one. Part of writing dialogue includes adding the subtext that indicates what the speaker is thinking. For help in learning how to add this subtext, go to: http://chasharrisfootloose.wordpress.com/2013/05/16/dialogue-with-subtext.