If you have a large numbers of Salesforce users, you can specify someone else to manage a sub-set of users as well. This person would be "delegated admin". e.g., you can specify that Mr. X can manage users of SalesManagers and Subordinates, so Mr. X becomes delegated user admin.
Here is the documentation : https://help.salesforce.com/HTViewHelpDoc?id=delegating_user_administration.htm&language=en_US (https://help.salesforce.com/HTViewHelpDoc?id=delegating_user_administration.htm&language=en_US)