In the US, if a person enters a guilty plea, a judge may proceed to convict and sentence the accused without any form of trial.
In the case of minor offenses with possible penalties of less than six months in jail, there is no US constitutional right to a jury trial. and the accused may be convicted and sentenced after a bench trial with no jury.
In the case of infractions that are not criminal, such as many traffic offenses, a judge or magistrate may make a judgment and impose a penalty after a brief and often informal hearing.
In some non-US jurisdictions, there is no right to a jury trial even in serious cases. In most such jurisdictions some form of due process and some hearing or trial is required for conviction.
However, in various authoritarian regimes, people may be "convicted" of "crimes" without anything like a trial before an independent tribunal.
In short, this depends on the nature of the case, and the laws of the jurisdiction involved.