A credit freeze is a tool to prevent ID theft for those people who believe that their information has been compromised. If an ID thief applies for credit and your report is frozen, the thief will be denied.
Each state deals with credit freezes differently. In some states there are fees, in others credit freezes are only available to consumers who have filed an ID theft related police report. Other states do not allow credit freezes at all.
To find out what the laws are in your state, check out Consumer’s Union for an up to date round up of credit freeze laws.