To invoke Apex classes to run at specific times, first implement the Schedulable interface for the class. Then, schedule an instance of the class to run at a specific time using the System.schedule method.

global class SomeClass implements Schedulable {
    global void execute(SchedulableContext ctx) {
        // awesome code here
    }
}

The class implements the Schedulable interface and must implement the only method that this interface contains, which is the execute method.

The parameter of this method is a SchedulableContext object. After a class has been scheduled, a CronTrigger object is created that represents the scheduled job. It provides a getTriggerId method that returns the ID of a CronTrigger API object.

Sample Code

This class queries for open opportunities that should have closed by the current date, and creates a task on each one to remind the owner to update the opportunity.

global class RemindOpptyOwners implements Schedulable {

    global void execute(SchedulableContext ctx) {
        List<Opportunity> opptys = [SELECT Id, Name, OwnerId, CloseDate 
            FROM Opportunity 
            WHERE IsClosed = False AND 
            CloseDate < TODAY];
        // Create a task for each opportunity in the list
        TaskUtils.remindOwners(opptys);
    }   
}

Using the System.Schedule Method

After you implement a class with the Schedulable interface, use the System.Schedule method to execute it. The System.Schedule method uses the user's timezone for the basis of all schedules, but runs in system mode—all classes are executed, whether or not the user has permission to execute the class.

Use extreme care if you’re planning to schedule a class from a trigger. You must be able to guarantee that the trigger won’t add more scheduled job classes than the limit. In particular, consider API bulk updates, import wizards, mass record changes through the user interface, and all cases where more than one record can be updated at a time.

The System.Schedule method takes three arguments: a name for the job, a CRON expression used to represent the time and date the job is scheduled to run, and the name of the class.

RemindOpptyOwners reminder = new RemindOpptyOwners();
// Seconds Minutes Hours Day_of_month Month Day_of_week optional_year
String sch = '20 30 8 10 2 ?';
String jobID = System.schedule('Remind Opp Owners', sch, reminder);

More info on this scheduling format here.

Scheduling a Job from the UI

You can also schedule a class using the user interface.

From Setup, enter in the Quick Find box, then select Apex Classes.

Apex
Click Schedule Apex.
For the job name, enter something like Daily Oppty Reminder.