A fixed-term tenancy is simply a tenancy agreement that states that it will end upon a specific date in the future, usually about six months or a year from the start of the tenancy. A periodic tenancy has no set end day and usually continues from month to month until one of the parties chooses to terminate the tenancy.
This leads to another important distinction: fixed-term tenancies are created by lease agreements while periodic tenancies are created by rental agreements. Therefore, the title of your agreement will change depending on your choice.
Many states give fixed-term tenants different rights than periodic tenants. This most commonly impacts the tenant's rights upon terminating the agreement or being evicted. The proper procedure for canceling a fixed-term or periodic tenancy depends on the terms of the agreement and state law.