A dismissal will be unfair if it is for a reason which does not fall into the categories of misconduct, poor performance, ill health or redundancy.
It will then only be fair if the employer has followed a reasonable procedure, and if the dismissal is within the range of reasonable responses open to an employer given all the circumstances of the case.
There is a significant amount of case law governing the circumstances in which a dismissal is fair, but overall it is a question of reasonableness, both in terms of the procedure and the decision to dismiss.