How can I keep segment leads/contact info so that a single users does not see all leads/contacts in the entire system?

How can I keep segment leads/contact info so that a single users does not see all leads/contacts in the entire system?

We have a contract rep doing some telesales, and need to put him on the system and provide him with lead lists, but need to make sure he can't see all of our other contacts, etc.  Is that possible, and if so, how?