From your switch name, I am assuming that you're using 3550 model.
Check out this link.
http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.1_9_ea1/configuration/guide/swtrafc.html#wp1042596
protect-When the number of port secure MAC addresses reaches the maximum limit allowed on the port, packets with unknown source addresses are dropped until you remove a sufficient number of secure MAC addresses to drop below the maximum value.
•restrict-A port security violation restricts data and causes the SecurityViolation counter to increment.
•shutdown-The interface is error-disabled when a security violation occurs.
