Carolyn, if you just “google” what you are looking for, you can find it. For example: http://www.geocities.com/gsmmobilereport/mobilemanagement.htm
In short, LAC (Location Area Code) is an identifier for a geographic area. Location updates are performed to let the network know where the mobile stations are in case of a MTC. If someone tries to call you, the network will send a “paging request” on the whole LAC where your mobile has last been registered, to try to contact you. Your mobile will then contact the network using a RACH to make the connection complete between the person calling you, the network, and yourself.
I hope this helps…