In order for the QueryTask object to execute a query against a layer in a map service you need to define the query in a Query object.
The Query object will specify whether the query will be spatial, attribute, or a combination of the two.
Attribute queries can be defined by either the where or text properties. These properties are used to define a SQL attribute query. We'll look at the difference between Query.where and Query.text in a bit.
Spatial queries require that you set the Query.geometry property to define the input geometry that forms the basis of the spatial query.
Creating a new instance of the Query object is as simple as calling its constructor with no parameters:
var query = new Query();