ELISA tests come in several forms, including direct, indirect, sandwich, and competitive ELISAs. Direct ELISA detects antigens directly, while indirect ELISA uses a secondary antibody to detect antibodies. Sandwich ELISA is used to detect specific antigens in complex samples, and competitive ELISA measures the competition between a labeled antigen and an unlabeled one for a limited number of antibodies. Each type has its advantages and is chosen based on the specific application's requirements.