An estate for years is a written agreement that has a definite start date and end date. This lease does not require notice from the tenant or landlord. At the conclusion of the term, the tenant is expected to move out.
An estate at will, om the other hand, is one that does not have a written lease. The tenant has permission to occupy the property for an indefinite period of time. The tenant or landlord can terminate the occupancy arrangement at any time or at will.