Both tenants and landlords have rights and obligations, whether they have signed a written lease or not. Nonetheless, a written rental agreement or lease that clearly specifies what a landlord and a tenant agrees to when renting a property is a good idea.
A rental agreement should stipulate the obligations of both the landlord and tenant(s). This will include when rent is due, who is responsible for utilities and other costs, and who is responsible for maintenance of the premises and the property.