If you are a SysAdmin user, you will see a Lock Record link at the top of Accounts, Contacts, Opportunities, and Cases (Pro) along with the lock/unlock icon (/).

When you lock a Record, the record will become un-editable by any user that does not have SysAdmin authority level. The SysAdmin users will still be able to edit the record and un-lock the record if needed. The lock applies just to the record you are working with. For instance, if you lock an Account, it does not automatically lock all the accounts contacts etc...

This gives you the ability to allow open access to your data but still be able to lock certain records if needed. You will be able to control read-only access to your record without having to go to the full Account Teams.

For instance, if you have opportunities that you would not like to be edited after they have been won or closed. You could lock them and they would no longer be editable by non SysAdmin users.

For more information on SysAdmin user authority, see this FAQ: http://www.salesjunction.com/afm/article.asp?fid=29

For more information on Account Teams, see the online documentation.