Defense attorneys always tell you the same thing, ALWAYS plead "NOT GUILTY." Generally a not-guilty plea will buy you some time to see what evidence the state is planning to use against you. But eventually, if an offer is made, you will likely consider pleading. Here are the differences:
Guilty
This one is a bit of a no-brainer. If you plead guilty, you're admitting to all of the facts being alleged against you. Generally, as I stated, don't ever do this at the onset of your case, unless God himself has come down from the heavens and convinced you of free entry upon a guilty plea. Even then, let him know you have to consult with your lawyer first.
No Contest
There are many question surrounding the No-Contest plea. What's the difference? What are you admitting to? Well, Generally a no-contest plea is helpful in one typical scenario: something most heinous has occurred, for which you will likely also be sued in civil court. If you cop to a guilty plea in court, you can bet your bottom dollar that plea will be used against you in the civil realm when someone is after you for money. A No Contest plea says to the judge, "I understand the evidence against me and the likelihood of a guilty verdict based upon the evidence. I accept my punishment but I do not admit guilt."
The Alford Plea
This is a bit of a unicorn. I doubt most have heard of the Alford plea, and even fewer have plead it. I had the fortunate experience of witnessing a man accused of a few hundred thousand dollars in the theft of farm equipment, who was also facing other serious charges, cop to an Alford Plea when I was in my final year of law school. Needless to say, I was stunned because this plea is not something generally taught in law school.
To put it simply, an Alford Plea is a plea of guilty, coupled with a claim of total innocence while, similarly to No Contest, admitting to the mountain of evidence against you, and the likelihood of a guilty verdict. One aspect of accepting a plea deal is that one is required to plead guilty in open court. For someone who insists on professing her innocence, she isn't likely to accept a guilty plea in open court. Hence, this rarified plea which allows acceptance of a plea deal while maintaining innocence. However, this plea isn't without its risk. That risk kicks in during sentencing. Generally, judges want to know that a defendant is remorseful for their actions. An Alford plea does just the opposite. So, in theory, this could come back to bite someone.
Not Guilty
I've saved the best for last. Again, another no-brainer. This is the one all Defense Lawyers insist you plead. It kicks off the trial process and gives the parties the opportunity to lay their cards on the table. You need to know what the state's evidence against you is, and whether the facts being alleged are the actual facts. You need to consider any possible defenses you may have, and assess your case as a big picture.