Typically, professional dancers are hired to judge a dance competition. They observe all the dancers keenly and sit in front of all the audience.
Each judge writes down the score sheet for each routine by awarding points for various categories such as techniques, facial expressions, costumes, choreography, and overall impression.
They may also write comments on their score sheets or in some cases, they even record audio comments for the dancers and their choreographers.
After recording audio comments, they are typically given to the dancers via CD or DVD media to allow review of the performance while listening to real-time judge commentary at the same time.
The categories of adjudication vary from competition to competition, but commonly judged categories cover the aspects of showmanship technique which are execution of movements, proper form, and transition, costume, difficulty of the routine, music and choreography.
A judge can give marks out of 100. If a performer or a team gets 100, it is called the perfect score. Each judge gives their score and the total score is nothing but the addition of all the individual scores of judges.