What is Big Deal Alert in Salesforce?
Your organization can use alerts that automatically send an email notification for opportunities with large amounts. Customize this alert to send an email when an opportunity reaches a threshold. Your threshold consists of an opportunity amount and probability. For example, you may want to send an email to your team that an opportunity of $500,000 has reached a probability of 90%.
for documentation please refer the line "https://help.salesforce.com/HTViewHelpDoc?id=activating_big_deal_alerts.htm&language=en_US"
Steps to Create Big Deal Alert.
Go to Setup-->Customize-->Opportunities