*en.wikipedia.org/wiki/Push_email
basically it is called push because the server sends 'signals' to the phone , unlike other methods like imap/smtp where the phone have to periodically check for new email.
Blackberry support push email , but i think you'll need a paid blackberry account for that.(not 100% sure abt that)
You can have free push email with newer nokia s60 handsets and with your existing hotmail/gmail/whatever account.