An assignment occurs when a tenant transfers its entire interest in a leased property for the remainder of the term of the lease to another party. A sublet occurs when the tenant transfers anything less than its entire interest.
When an assignment occurs, tenants are normally released from all responsibility for the lease. However, it is important to note whether the terms of an assignment actually give a full release from all responsibility. Sometimes the language will keep the original tenant on the hook if the new tenant fails to pay rent or otherwise fails to fulfill its obligations.
On the other hand, with a sublease the tenant remains ultimately responsible if the subtenant fails to uphold its responsibilities. This is because, technically, there is no agreement between a landlord and a subtenant. The landlord must seek recovery against the original tenant. Often, subleases are temporary in duration, with the subtenant moving out of the property before the term of the original tenant's lease agreement expires.