A plea of guilty is an admission to the charge and will most likely result in a finding of guilt by the judge
A plea of no contest is similar to a plea of guilty and will also most likely result in a finding of guilt. However, with a no contest pleas, you admit to no civil liability in connection with the incident, such as a claim for damages incurred in an automobile accident.
A plea of not guilty means that you feel the charge against you is not correct, or you wish to make the court aware of any issues related to the citation.