Help with Compile Error: DML requires SObject or SObject list type: List<String>

public with sharing class QualOpController
{
public List <Opportunity> listOfOpty {get;set;}
public List<String> campaigns {get;set;}
public PageReference saveOp(){
    UPDATE campaigns;
    return null;
    }
 public String getName(){
        return 'QualOpController';
        }
public void load() 
{
  campaigns= new List<String>();
  listOfOpty = [Select id, name, CreatedDate, StageName, Vert_Med__c, Company_Name__c, Next_Step__c, Vertical__c, Weeks_Live__c, Days_Since_Last_Modified__c, Contract_SENT__c, NextStep, LeadSource, Probability, Spend_Last_Week__c, Spend_Last_30_Days__c, Owner_Name__c, Revenue_All_Time__c from Opportunity WHERE Pipeline_Oppty__c = TRUE ]; 
  Set<String> campaignSet = new Set<String>();
  for (Opportunity j : listOfOpty)
      campaignSet.add(j.Vert_Med__c);
      for (String campaign : campaignSet) 
      {
            campaigns.add(campaign);
      }
      campaigns.sort();
    }
}