A Section 21 notice is a ‘no fault’ notice, and therefore you do not need to give a reason as to why you want the tenant to leave, or why you want to recover possession of the property.
This is the most commonly used notice and has a required form (Form 6a). This can be used at the end of the fixed term in an AST, or during the rolling period in a Periodic Tenancy.
A Section 8 notice can be used whether or not the fixed term of the tenancy agreement has expired, and you wish to recover possession of the property. This is commonly used where the tenant is in rent arrears, or has demonstrated anti-social behaviour at the property.