Hi Luca,

Batch Apex:
A Batch class allows you to define a single job that can be broken up into manageable chunks that will be processed separately.

Here is how to write sample Batch Apex:
1) Start method is automatically called at the beginning of the apex job. This method will collect record or objects on which the operation should be performed. These record are divided into subtasks & passes those to execute method.

2) Execute Method performs operation which we want to perform on the records fetched from start method.

3) Finish method executes after all batches are processed. Use this method to send confirmation email notifications.

Reference:
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_batch.htm

The sample code would like below:

public class MyBatchable implements Database.Batchable<SObject> {

    public Database.QueryLocator start(Database.BatchableContext context) {
        return Database.getQueryLocator([select Id from Account order by Name]);
    }

    public void execute(Database.BatchableContext context, List<Account> scope) {
        Account[] updates = new Account[] {};
        for (AggregateResult ar : [
                select AccountId a, count(Id) c
                from Contact
                where AccountId in :scope
                group by AccountId
                ]) {
            updates.add(new Account(
                    Id = (Id) ar.get('a'),
                    No_of_Contacts__c = (Decimal) ar.get('c')
                    ));
        }
        update updates;
    }

    public void finish(Database.BatchableContext BC)
    {
        AsyncApexJob a = [Select Id, Status,ExtendedStatus,NumberOfErrors, JobItemsProcessed,TotalJobItems, CreatedBy.Email    from AsyncApexJob where Id =:BC.getJobId()];
      
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        String[] toAddresses = new String[] {Email to addresses};
        mail.setToAddresses(toAddresses);
        mail.setSubject('Account Number Email ');
        mail.setPlainTextBody('Account Number ' + a.Name);
        Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
    }
}

To run the Batchable:

Database.executeBatch(new MyBatchable(), 50);

This Batch apex will count number of Contacts associated with Account object.It will store the count in a field "No_of_Contacts" on Account.And sends an email with provided email addresses with provided content.

Additional Reference:
http://amitsalesforce.blogspot.com/2016/02/batch-apex-in-salesforce-test-class-for.html

Hope above information helps, Please mark as Best Answer so that it can help others in the future.

Thanks.