This allows you to send the workbook by email, routing server, internet fax service, etc. (Certain options may not be available on certain networks/systems). You get several choices from the sub-menu for all the different options available. (Below)

Clicking on any of the email options should open your email client, such as Outlook or Outlook Express, etc. In latter versions of Excel (XP and up), selecting 'Send to> Mail recipient' has opened a mailing header within Excel allowing you to send the workbook or just the worksheet directly from Excel.

 'Send email for review' requires that the workbook be shared, via Tools>Share Workbook...