In criminal cases there are three different pleas that can be entered. This post gives a short summary of each plea and what they accomplish in a criminal case.
A not guilty plea indicates, above all else, that you would like to take advantage of the adversarial process within the criminal justice system. A person may plead not guilty because they are innocent of the charges against them, they have a legally recognized defense, or they simply do not believe the charges against them can be proven beyond a reasonable doubt. More important than why a person pleads not guilty is what that plea allows them to do. By pleading not guilty an accused person can now take steps towards defending against the charges, whether that is by filing motions, participating in discovery, or having the case set for trial. In almost every case a person will start by entering a not guilty plea because that is the only way to truly fight/defend a criminal charge.
A guilty plea indicates to the court that you are admitting the allegations against you and waiving your rights to defend against the case. Before a judge can accept a guilty plea from a defendant the judge needs to ensure the defendant understands the rights that they have and that they are knowingly, intelligently, and voluntarily waiving those rights. Examples of those rights are the right to have a trial, the right to call witnesses in your defense, and the right to file motions in your case. An experienced criminal defense attorney can review in detail with you all of the rights you give up and advise you on whether or not entering a guilty plea is in your best interests.
No Contest a.k.a. Nolo Contendere
A no contest plea indicates to the court that you are neither admitting nor denying the allegations against you, but you are waiving your right to defend against the charges and accepting any punishment the court imposes. A no contest plea functions almost exactly the same way as a guilty plea with two key differences. 1) A no contest plea is NOT an admission, and therefore cannot be used against you as an admission in any other court proceedings; 2) it is up to the judge whether they will allow you to enter a no contest plea. The main advantage of a no contest plea is that it cannot be used against a person later as an admission of guilt or wrongdoing. This is especially important in criminal cases that have a related civil case.