Until now (July 2014), record locking mechanism in Salesforce only can be initiate from Approval Process. Some users will see Unlock Record button next to Edit button. Clicking that button will unlock the record, and the record become editable by record owner and standard users.
Compare with screenshot below when view by record owner or other user as below.
So, who will see the Unlock Record button and able to unlock the record:
1. User with System Administrator profile
2. User with Modify All Data permission
3. User with Modify All permission in the object locked
Once a record is unlock:
1. Pad lock icon will be disappear
2. Record owner or users with ability to edit the record will able to edit the record
3. In the object history, it will show date/time, user with action 'Record unlocked.'
4. The approval process for the record still in place, approver still able approve or reject the record.
Reference: Edit Locked Record