Trigger Context Variables

1) isExecuting Returns true if the current context for the Apex code is a trigger, not a Visualforce page, a Web service, or anexecuteanonymous() API call.
2) isInsert Returns true if this trigger was fired due to an insert operation, from the Salesforce user interface, Apex, or theAPI.
3) isUpdate Returns true if this trigger was fired due to an update operation, from the Salesforce user interface, Apex, or theAPI.
4) isDelete Returns true if this trigger was fired due to a delete operation, from the Salesforce user interface, Apex, or theAPI.
5) isBefore Returns true if this trigger was fired before any record was saved.
6) isAfter Returns true if this trigger was fired after all records were saved.
7) isUndelete Returns true if this trigger was fired after a record is recovered from the Recycle Bin (that is, after an undelete operation from the Salesforce user interface, Apex, or the API.)
8) new Returns a list of the new versions of the sObject records.Note that this sObject list is only available in insert and update triggers, and the records can only be modified in before triggers.
9) newMap A map of IDs to the new versions of the sObject records. Note that this map is only available in before update, after insert, and after update triggers.
10) old Returns a list of the old versions of the sObject records.Note that this sObject list is only available in update and delete triggers.
11) oldMap A map of IDs to the old versions of the sObject records.Note that this map is only available in update and delete triggers.
12) size The total number of records in a trigger invocation, both old and new.

Sample trigger

trigger AccountTrigger on Account( after insert, after update, before insert, before update)
{

    AccountTriggerHandler handler = new AccountTriggerHandler(Trigger.isExecuting, Trigger.size);
    
    if( Trigger.isInsert )
    {
        if(Trigger.isBefore)
        {
            handler.OnBeforeInsert(trigger.New);
        }
        else
        {
            handler.OnAfterInsert(trigger.New);
        }
    }
    else if ( Trigger.isUpdate )
    {
        if(Trigger.isBefore)
        {
            handler.OnBeforeUpdate(trigger.New ,trigger.Old,Trigger.NewMap,Trigger.OldMap);
        }
        else
        {
            handler.OnAfterUpdate(trigger.New ,trigger.Old,Trigger.NewMap,Trigger.OldMap);
        }
    }
}

Create one Trigger Handler Class

public with sharing class AccountTriggerHandler 
{
    private boolean m_isExecuting = false;
    private integer BatchSize = 0;
    public static boolean IsFromBachJob ;
    public static boolean isFromUploadAPI=false;
    
    public AccountTriggerHandler(boolean isExecuting, integer size)
    {
        m_isExecuting = isExecuting;
        BatchSize = size;
    }
            

    public void OnBeforeInsert(List<Account> newAccount)
    {
        system.debug('Account Trigger On Before Insert');
    }
    public void OnAfterInsert(List<Account> newAccount)
    {
        system.debug('Account Trigger On After Insert');
    }
    public void OnAfterUpdate( List<Account> newAccount, List<Account> oldAccount, Map<ID, Account> newAccountMap , Map<ID, Account> oldAccountMap )
    {
        system.debug('Account Trigger On After Update ');
        AccountActions.updateContact (newAccount);
    }
    public void OnBeforeUpdate( List<Account> newAccount, List<Account> oldAccount, Map<ID, Account> newAccountMap , Map<ID, Account> oldAccountMap )
    {
        system.debug('Account Trigger On Before Update ');
    }

    @future 
    public static void OnAfterUpdateAsync(Set<ID> newAccountIDs)
    {

    }      
    public boolean IsTriggerContext
    {
        get{ return m_isExecuting;}
    }
    
    public boolean IsVisualforcePageContext
    {
        get{ return !IsTriggerContext;}
    }
    
    public boolean IsWebServiceContext
    {
        get{ return !IsTriggerContext;}
    }
    
    public boolean IsExecuteAnonymousContext
    {
        get{ return !IsTriggerContext;}
    }
} 

Create one Trigger Action Class

public without sharing class AccountActions 
{
    public static void updateContact ( List<Account> newAccount)
    {
        // Add your logic here
    }
}