When you create an alert, you can choose to have the results of your alert emailed to you either daily or immediately.

• Once Daily: Each day, at your selected time(s), you receive an email that contains a list of all the items matching your alert specifications. If more than 20 items match, only the latest 20 are sent to you. We recommend this option if you do not wish to receive a large amount of email.
• As they happen: With this option, you receive an email every time an item matches your alert criteria. Select this option only if immediate notification is critical. Each item that is found is sent in a separate email, so you might receive a large amount of email.