div-ajax-load

ajax.js

//jquery

$.ajax({
   url : 'seu-arquivo.html',
   success : function (data){
       $('.ajax').html(data);
   },
   error : function(){
       $('.ajax').html('Erro ao carregar conteúdo...');
   }
});

//js puroo
var req = new XMLHttpRequest();

req.open('GET', 'seu-arquivo.html');
req.onload = function(){
    var div = document.getElementById('ajax');
    div.innerHTML = req.response;
};

req.send();