If you and your landlord make an agreement, but do not write it down, you have an oral lease. The rental period begins on the day your rent is due. You are not bound for a certain amount of time after this period. An oral lease on a month-to-month unit allows you to move out with just a one month's notice. An oral lease also allows the landlord to refuse to continue to rent to you with one month notice. Additionally, a landlord can increase the rent with a written letter giving you a full rental period's notice. The landlord may also add rules that you may not like. Many tenants feel that they have a lot less power if they do not have a written lease. You are probably more stable in your housing if you have a written lease.
Written leases
Written leases clearly state the policies to which you and your landlord agree. You can take the lease home with you and have a knowledgeable person look it over for you. You can also negotiate the terms of the lease with the landlord. Make sure that both you and the landlord initial any changes made to the lease.
If you have a dispute with your landlord your lease is still in effect. You may feel that you have cause to stop paying rent, but don't (unless you have been advised to stop by an attorney). If you believe you have reason to withhold rent, talk to a lawyer.