A tenancy agreement is a contract agreement between a tenant and a landlord. It may be written or oral. It defines the legal relationship between a tenant and a landlord, for example, the tenant right to occupy an accommodation and the landlord's right to receive rent for letting the accommodation.
A tenancy will generally take the form of a fixed-term, which is created for a specified length of time - 6 or 12 months. During this period, the tenant must pay rent for the full term and the landlord cannot regain possession during this period.
When tenants stay on after the end of the fixed-term, the tenancy will automatically (the landlord does not need to do anything) become a statutory periodic tenancy – which is a tenancy protected by the statutory rules of the Housing Acts 1988 & 1996, running in periods based on the rent payment periods - month to month for a monthly tenancy. With a periodic tenancy the landlord does not have the security of a longer term let.
A tenancy does not end when the contract runs out. It becomes a periodict tenancy, as opposed to the fixed term tenancy you had originally. This means it now runs from month to month rather than for a specific period. In other words you now occupy under a series of one-month tenancies, or one-week, or 4 weekly tenancies, if you pay rent weekly or every 4 weeks.
Most of your rights and responsibilities will remain the same and the original tenancy agreement remains valid and in force, but the rules on ending the tenancy is different. If you decide to leave it's important you end your tenancy correctly otherwise you could still be liable to pay rent even though you have left.