A bull terrier is a chain match. Each participant wears a collar around their neck, which has a chain to connect to their rival.
To win a bull terrier match, a luchador must touch all the corners (4 or 6) of the ring, starting with any corner but continuing in order from there. If the luchador is interrupted in touching the corners, the count restarts. It's up to the referee to determine if the count has been interrupted and what counts an interruption varies from match to match.
Bull terrier matches are staples of AAA and indy groups, and are not done in CMLL. The basic format is a mano a mano match, but their are variants for triangle and teams.