Just like in anything, defense will not give you a win.  In self defense, you want to jump to offense as soon as possible.  Now some times you know when the opponent is about to attack and can attack them first (preemptive attacks), but this is not always the case.  Sometimes the opponent strikes and you have that split moment to stop them before the strike gets to you.

This is where a defense comes in.  You have three basic defenses to choose from: Blocking, parrying, and defensive attacks.

Blocking is exactly what it sounds like:  putting a guard up for the opponent to hit instead of their target.  Blocking is the easiest way to keep the opponent from hitting vital areas, but it is very hard to jump to an offense from it.

Parrying is a way of deflecting the opponent's strike.  Most commonly, it includes slapping the opponent's punch off line.  This is very effective, and puts you in good position to attack.  The only thing you have to worry about is missing the opponent's strike and still getting hit.

Defensive attacks are attacks to stop the opponent's attack and automatically be in an offensive position.  These are usually strikes to the eyes, to the attacking arm, or pushes.  defensive attacks are great for transitioning to offense, but if they are not successful, you will most likely get hit.  Another advantage to using these:  they are reletively new to the martial arts world.

Defense is used to stop an initial strike and jump into offense.  Using parrys and defensive attacks are the best for this, blocking should be used as a last resort.