A plea bargain is an "offer" to the defendant in a criminal case. The "offer" from the government in "in exchange for" a plea of guilty (sometimes "no contest" is allowed) to charges associated with the defendant's case. If a plea bargain is obtained, then no trial will take place. The plea bargain will be considered a binding agreement, and the defendant will have to abide by the terms that he/she entered into.
For example, if Henry is charged with the felony of "Assault with a Deadly Weapon" (CA Penal Code section 245) in San Diego after being involved in a fight with Kelly, the San Diego Deputy District Attorney may offer Henry a plea bargain. For this example, let's say the maximum penalty for Henry's charge is 4 years in state prison. The District Attorney may offer Henry a deal that would keep in out of state prison. The offer could be that Henry would have to plead guilty, and then he would be placed on probation for 3 years, ordered to serve 180 days in county jail, and be ordered to pay restitution to Kelly for her injuries.
It is completely up to the client/defendant to decide whether or not he/she wants to enter into a plea bargain if one if offered. It is the sole choice of the client/defendant to decide whether or not to take his/her case to trial. It is important to discuss all options with your attorney prior to entering into a plea agreement.