co-signer vs. guarantor

A lease co-signer, by name, co-signs the lease.  He is bound by the lease, and liable not only to rent, as well as damages to the unit.  A co-signer has a right to move into the unit just as other tenants.  

A guanrantor only gurrantees certain financial aspect of the lease, such as rent, but not damages.  Guarrantor do not have access to the unit.  He cannot move into the unit without approval.  Because guaranttor is not bound by the lease, you need a separate agreement for guarantor to define the scope of his guarrantee.  Google "GUARANTOR AGREEMENT (Addendum to the Lease rental Agreement)"

As an example, a student might find his parents to be a co-signer, i.e. responsible to ALL aspect of the lease, such as rent, late fees, damages to the unit, penalties for violation (such as unauthorized pets, party etc).  

On the other hand, the student's uncle/friend/girlfriend might not be comfortable for such broad guarrantee.  Thus, an uncle/friend/girlfriend might only agree to be his guarrantor, i.e. only responsible for rent but no more.