An assignment of a lease transfers the tenant's entire interest in the lease to someone else. Under a sublease the subtenant typically pays something more or gets something less than is otherwise given in the lease between the tenant and the landlord.
Unless prohibited by the lease, the lease may be assigned or sublet without terminating it. The first tenant remains fully responsible for all of the obligations of the lease. Neither assignment nor subletting releases the tenant from liability to the landlord. If the lease requires the landlord's permission to assign or sublease, the landlord cannot arbitrarily or unreasonably reject a substitute tenant. The withholding of consent must be reasonable.