First and foremost, all skaters should be aware that judges are all volunteers who have spent countless hours of their personal time qualifying to be judges and continue to volunteer their time as both testing and competition judges.
There are two different judging systems used for competitions: 6.0 and the International Judging System (IJS).
For the 6.0 system, there will always be an odd number of judges, usually three, and each skater will be ranked by majority within their skating group. So, if there are 5 skaters in a group, each judge will rank each skater from 1 to 5, as compared to each other. The skater with the majority of 1st place rankings will be the winner, the skater with the majority of 2nd place rankings will be 2nd, etc.
The 6.0 system is generally used for Compete USA events and many nonqualifying events, especially for pre-juvenile levels and below.
The IJS is what most people are probably familiar with as it is used at all national and international competitions including the Olympics. With this system, skaters are judged in two areas: technical skill and program component. For the technical skill score, skaters receive points based on the degree of difficulty of each technical element performed in their program (base points) as well as how well each element was executed (Grade of Execution or GOE). The GOE can be either positive, if the element was performed above the standard, or negative, if it was performed poorly (think under-rotation on jumps). The program component score earns the skater points for their overall skating ability and performance level.
IJS is used for all qualifying competitions and many nonqualifying competitions at the juvenile through senior levels. It is also used for all Excel Plus programs and all juvenile through senior level singles programs at Excel competitions. It may also be used for other events at nonqualifying competitions, but it is not required. The competition hosts for nonqualifying competitions make the decision whether to use 6.0 or IJS, so always review the competition rules which will clearly indicate which type of judging will be used for each event.