the contract

When a person occupies a property and pays rent a tenancy is automatically created. You no longer have the right to occupy the property and can only visit with the tenant’s permission. To remove that person now requires the actions of a court. The created tenancy is governed by the Housing Act 1988.
Having a tenancy created without any paperwork to back it up is not wise. Both the landlord and tenant have rights but without anything in writing there is no record as to how long the tenancy is for or how much the tenant is expected to pay. Any dispute will be very hard to resolve especially by the courts as it is only each parties word against the other.
The tenancy agreement solves this problem as everything is in writing and details legally what is expected of both the landlord and the tenant from timescales for staying in the property and how much rent is paid to what the tenant can do in terms of personalising the property. Anything can be included in the agreement but whatever is written cannot override housing law.

This is the default tenancy in England and Wales. This occurs even without an agreement being provided in writing. AST’s are the default agreements but there are others which are not as landlord friendly and the AST would have to be deliberately overridden to create one. Details of what happens in Scotland is contained elsewhere. At some point (no date published yet) the Renting Homes (Wales) Act 2016 will come into effect. This replaces AST’s with Standard Contracts which will differ in some important ways.

When the rent is in excess of £100k per year.
When the tenant is living within the same building as the landlord but in a self-contained area.
When the property is only used occasionally by the tenant such as weekends.
When the property is a holiday let.
When the tenant is a limited company.
When the tenant is living with the landlord (i.e. as a lodger).
With holiday lets and lodgers the tenant has no security of tenure and must leave at the end of the agreement. A lodger for example could be allowed to stay for a month with a verbal agreement and asked to leave at the end of this time. You could still have a written agreement, but it wouldn’t be an AST – it would be a license agreement. The same applies to holiday lets – the occupant must leave when requested to do so.
If the tenant is a limited company a contractual agreement would ned to be signed. In this case the Housing Act 1988 rights don’t apply. The landlord can then be more flexible in the terms of the contract. The tenant will still be protected under the Unfair Terms in Consumer Contracts
Regulations 1999. The landlord will still need to follow the Landlord and Tenants Act 1985 in regard to repairing obligations.