iPod touch What does fetch new data mean and push?

Fetch new data is how often it checks your emails, so every 15, 30 or hourly, and manual only checks when you open the email app or hit refresh in the email app.
Push is a type of email that, well as soon as you get an email, it is "pushed" to your ipod. I beleive push only works with Mobile Me, i could be wrong, but if you do not have push email, then turn it off to save battery life