Explain about salesforce workflow and when to use Salesforce workflow

A workflow rule is triggered based upon pre-defined criteria. The workflow rule triggers workflow actions which can update fields, send emails, task create and outbound message.

Workflow Rules Criteria:

When a record is created, or when a record is edited and did not previously meet the rule criteria
Only when a record is created
Every time a record is created or edited (may not be used in conjunction with time-based workflow actions)

Filter criteria must be entered as well, same functionality as a report but is required.

Workflow actions can do one of the following:

Create tasks
Send emails. There are some limitations here- for instance, whoever activates the workflow rule will be the sender of the email (from cannot be set).
Update fields. There are some limitations here- for instance, you cannot update a lookup field.
Send outbound message (used to send information to external services).

Workflow actions are either: