trigger OpportunityValidation on Opportunity (before update) {
    for (Opportunity opp : Trigger.new) {
        if (opp.StageName == 'Closed Won' && String.isBlank(opp.Description)) {
        opp.addError('Description is required before closing an Opportunity.');
        }
    }
}

Prevents an Opportunity from being closed without a description.

public class AccountUpdater {
    public static void normalizeAccounts() {
        List<Account> accs = [SELECT Id, Name FROM Account WHERE Name LIKE 'Test%'];
        for (Account a : accs) {
        a.Name = a.Name.replace('Test', '');
        }
        update accs;
    }
}

Custom Apex class that queries Accounts and updates a field based on logic.