What is Scheduler class in Salesforce Apex and explain how to use Cron statement to Schedule Apex class?

The Apex class which is programmed to run at a predefined interval.

Class must implement schedulable interface and it contains method named execute().

There are two ways to invoke schedular:

Using UI
Using System.schedule

The class which implements interface schedulable get the button texted with “Schedule”, when user clicks on that button, new interface opens to schedule the classes which implement that interface.
To see what happened to the scheduled job, go to “Monitoring | Scheduled jobs ”

Example of scheduling:

scheduledMerge m = new scheduledMerge();
String sch = '20 30 8 10 2 ?';
system.schedule('Merge Job', sch, m);