Workflow rules can help automate the following types of actions based on your organization's processes:


Tasks: Assign a new task to a user, role, or record owner.
Email Alerts: Send an email to one or more recipients you specify.
Field Updates: Update the value of a field on a record.
Outbound Messages: Send a secure, configurable API message (in XML format) to a designated listener.
For example, workflow rules can:
Assign follow-up tasks to a support rep one week after a case is updated.
Send sales management an email alert when a sales rep qualifies a large deal.
Change the Owner field on a contract three days before it expires.
Trigger an outbound API message to an external HR system to initiate the reimbursement process for an approved expense report.
Each workflow rule consists of:
Criteria that cause the workflow rule to run.
Immediate actions that execute when a record matches the criteria. For example, salesforce.com can automatically send an email that notifies the account team when a new high-value opportunity is created.
Time-dependent actions that queue when a record matches the criteria, and execute according to time triggers. For example, salesforce.com can automatically send an email reminder to the account team if a high-value opportunity is still open ten days before the close date.

For ref : https://wiki.developerforce.com/page/Workflow_Rules

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Subramani,
Trinay technology Solutions