This setting lets you turn Push on or off for MobileMe, Microsoft Exchange, Yahoo! Mail, and any other push accounts you've configured on iPhone. Push accounts automatically deliver new information to iPhone when new information appears on the server (some delays may occur). You might want to turn Push off to suspend delivery of email and other information, or to conserve battery life.

When Push is off, and with accounts that don't support push, data can still be fetched—that is, iPhone can check with the server and see if new information is available. Use the Fetch New Data setting to determine how often data is requested. For optimal battery life, don’t fetch too frequently.

Turn Push on:  
Choose "Mail, Contacts, Calendars" > Fetch New Data, then tap to turn Push on.

Set the interval to fetch data:  
Choose "Mail, Contacts, Calendars" > Fetch New Data, then choose how often you want to fetch data for all accounts.

To conserve battery life, fetch less frequently.

Setting Push to OFF or setting Fetch to Manually in the Fetch New Data screen overrides individual account settings.