upper limit (circuit) is the maximum price at which a particular share can quote for buying / selling. Lower limit (circuit) is the minimum price at which a particular share can quote for buying / selling. If a scrip has quoted Rs.100/- at the end of 8.9.11, and the circuit limit is 5%, on 9.9.11, the scrip can be traded between Rs.95/- to Rs.105/- only. That is + 5% and -5%. The percentage will be decided by Stock Exchange. % may be different for each scrip.