Your email for example. Through the gmail app anytime you have a new email, gmail will push the email and notify you of that email.
If you use the default email app for you email account, and it's set up to polling, the app will check every 15/30/60 minutes for emails. (IMAP and/or POP)
The latter takes alot more power to do. The same concept applies for any application/service that fetches data from a server.
Hotmail, supports push through activesync and their own app. Yahoo supports push through their own app. Gmail is push through their own app.
If you have to use IMAP / POP for your emails set the polling intervals higher, if you don't NEED to have constant updates for the email.