Tasks in the Spatial Analysis service execute asynchronously, meaning that you submit a request (or job) and then poll the service for status. When the status is esriJobSucceeded, you then access the results of the task. When you submit a request, the Spatial Analysis service returns to you a unique job ID for the transaction.
http://<analysis url>/arcgis/rest/services/tasks/GPServer/<task name>/jobs/<jobId>
You can use the job ID to periodically check the status of the job and messages. Additionally, if the job has successfully completed, you can use the job ID to retrive the results.
The jobStatus property can have the following values: