An abridged version of any text is a shortened version of the original. The differences between the abridged version and the original always depends on exactly how much of the original text was cut out, and that will always vary per the decisions of the editors. Some abridged versions of texts maintain 70% of the original, whereas others only maintain 25%. Looking at some of the abridged versions of Shakespeare's Romeo and Juliet, many editors have chosen to cut performance time of the play down from about 2 hours to about 30 to 45 minutes, which would roughly maintain between 28% to 37% of the original text, which is definitely very little of the original text.
Looking at one version edited by Shawn Peters, we see that he maintained the original Shakespearean language and expressed the main points of all scenes. However, a lot of the dialogue has been cut out, and many of those lines reveal essential character traits that serve to help develop Shakespeare's themes; lines cut out also serve to develop the philosophical views Shakespeare expresses in the play.
One example of what seems to be a simple line of dialogue cut out of the original but actually reveals a great deal about Tybalt's character can be seen the opening scene of the abridged version. The abridged version immediately starts with the servants of the Capulets and Montagues beginning a fight and Benvolio trying to bring peace by beating down their swords. Tybalt immediately joins the fray, saying, "Turn thee, Benvolio, look upon they death." Yet, in the original, Tybalt's opening line is, "What, art thou drawn among these heartless hinds?" (1.1.52). While it seems like such a small change, Tybalt's real opening line reveals just how quick Tybalt is to misjudge a situation and to react violently, two critical character traits that lead to his downfall.
In addition, critical opening lines spoken by the prince are cut out from the original in the abridged version. The prince's opening speech reads as follows in the original:
Rebellious subjects, enemies to peace,
Profaners of this neighbour-stained steel,--
Will they not hear? (1.168-70)
The phrase "neighbour-stained steel" is especially key to expressing one of Shakespeare's central points: these are neighbors, not enemies from distant shores, that are causing so much death and destruction due to persistence in acting upon their irrational emotions rather than their sound reasoning.