difference between esri geoprocessor and <esri:QueryTask

[NadeemQazi]

I am trying to execute the a geo processing task from flex api. i have seen examples provided by the user. one of the examples
given on the link http://resources.arcgis.com/en/help/flex-api/samples/index.html#/Query_using_GP_result/01nq0000003t0...
has  used both

<esri:Geoprocessor id="gp"
                           concurrency="last"
                           executeComplete="onGPExecuteComplete(event)"
                           outSpatialReference="{myMap.spatialReference}"
                           showBusyCursor="true"
                           url="http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/Network/ESRI_DriveTime_US/GPServer/Create..."
                           useAMF="false"/>

        <esri:QueryTask id="queryTask"
                        concurrency="last"
                        executeComplete="onQueryExecuteComplete(event)"
                        showBusyCursor="true"
                        url="http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/Demographics/ESRI_Census_USA/MapServer/0"
                        useAMF="false"/>

my question what 's difference between these two and if make a model builder and publish it as service than which one of the above should be used esri geoprocessor tag or esri querytask.
regards
nadeem