trigger CategorizedOpprtunity on Opportunity (before insert, before update) {
    for (Opportunity opp : Trigger.New) 
    {
        if (opp.Amount != null)
        {
            if (opp.Amount < 10000) 
            {
                opp.Category__c = 'Small';
            } 
            else if (opp.Amount >= 10000 && opp.Amount < 50000)
            {
                opp.Category__c = 'Medium';
            } else if (opp.Amount >= 50000) {
                opp.Category__c = 'Large';
            }
        }
    }
}