Writing a Timer Job

Scenario:
You want to run some code on a regular interval and decided to write a timer Job.

Sample Timer Job Code:

using System;
using System.Net;
using Microsoft.SharePoint;
using Microsoft.SharePoint.Administration;

namespace SharePoint.TrainingSamples {

public class SharePointTimerJob:SPJobDefinition {

   public SharePointTimerJob():base(){ }

   public SharePointTimerJob(string jobName,SPWebApplication webApplication)
      :base(jobName, webApplication, null, SPJobLockType.ContentDatabase) {
      this.Title = "MyTimerJob";
  }

public override void Execute (Guid targetInstanceId) {
 foreach (SPSite siteCollection in this.WebApplication.Sites) {
   // Code to be executed as a part of Timer Job
foreach (SPSite siteCollection in this.WebApplication.Sites)
           {
               SPWebApplication webApplication = this.Parent as SPWebApplication;
               SPContentDatabase contentDb = webApplication.ContentDatabases[contentDbId];

               // get reference to "Tasks" list in the RootWeb of
               // the first site collection in the content database
               SPList taskList = contentDb.Sites[0].RootWeb.Lists["Tasks"];

               // create a new task, and update item
               SPListItem newTask = taskList.Items.Add();
               newTask["Title"] = DateTime.Now.ToString();
               newTask.Update();

           }
 }
}
}
}

Feature Activation Code:
Now that timer job is ready you need to add it to SharePoint Timer Jobs Store, One way is to add it as a part of Feature Activation

// get a reference to our job class in the GAC
public override void FeatureActivating (SPFeatureReceiverProperties properties) {

SPSite site = properties.Feature.Parent as SPSite;

//This is the timer class you have created above
SharePoint.TrainingSamples.SharePointTimerJob oSharePointTimerJob = new SharePoint.TrainingSamples.SharePointTimerJob("MyTimerJob", site.WebApplication);
// set the execution schedule to every 5 minute
SPMinuteSchedule schedule = new SPMinuteSchedule();
schedule.BeginSecond = 0;
schedule.EndSecond = 59;
schedule.Interval = 5;
oSharePointTimerJob.Schedule = schedule;
//update the job
oSharePointTimerJob.Update();
}

Feature De-Activation Code:
You might want to remove it from SharePoint Timer Jobs Store on de-activation

public override void FeatureDeactivating (SPFeatureReceiverProperties properties) {
 SPSite site = properties.Feature.Parent as SPSite;

 // Delete the job.
 foreach (SPJobDefinition job in site.WebApplication.JobDefinitions) {
   if (job.Name == "MyTimerJob")
     job.Delete();
 }
}

Articles:
MUST READ : Issues with Timer job , OTHER Helpful links MSDN , Must Read , Andrew Connell