@35: Here's my attempt at a less-jaded answer. For each crime, the legislature sets the maximum sentence, and for felonies, it also sets the standard range for each crime, depending on your criminal history. For gross misdemeanors, like DUI or theft under $750 or simple assault, the statutory maximum is 364 days in jail (it used to be 365, but because the 1 year mark may trigger immigration consequences, that was changed a few years ago). A judge can impose anything under that max, unless it's a DUI, where there are some mandatory minimums (http://www.courts.wa.gov/newsinfo/conten…
For felonies, it's a little more complicated, because the sentencing framework set up by the legislature depends on both your crime and your criminal history. For example, for Assault 2 (i.e. breaking someone's bone, or assaulting them with a knife/gun), the max is 10 years. With no history, the standard range is 3-9 months; with at least 9 prior convictions, the standard range is 63-84 months (and there are 8 steps in between). A judge can rarely go above or below that standard range.
What Jason Lantz is talking about is that although the legislature sets the sentencing range, the prosecutor can influence what standard range a defendant faces by choosing what to charge or what to offer.
Part of the reason for the emphasis on plea bargaining is just the volume of cases going through the system. I don't know the exact numbers, but a SMALL percentage of defendants take their cases to trial (it's less than 10%). Despite that small number, the trial courts are busy all of the time. Without a high percentage of pleas, things would grind to a halt.