If you are looking for one quote from William Shakespeare's Romeo and Juliet which defines the setting, situation, conflict and the characters, one would be hard-pressed to provide a single quote which encompasses all information desired. That said, one need not look any further than "The Prologue" to find a summary of the play.
While the prologue does not contain a single quote, it does provide all of the information needed. When broken down, different parts of "The Prologue" provide both direct and indirect information about the play itself.
Two households, both alike in dignity,
In fair Verona
Lines one and two of "The Prologue" provide the information about the setting.
From ancient grudge break to new mutiny,
Here, in line three, the situation is given: two feuding families have a new fight.
From forth the fatal loins of these two foes
A pair of star-cross'd lovers take their life;
Lines five and six give the conflict: the children of the feuding families have fallen in love, and, in the end, take their lives.
While a direct naming of the characters is not given, "The Prologue" offers clues to the characters. As seen above, readers/watchers of the play can tell that the play involves two different families. The children of each family have fallen in love. Though unnamed at this point, the characters have been introduced.