What does 'push' data mean on the iphone?

Whenever I download a new app that has data, it can send push messages or texts or emails to my phone? What does it mean and what's the difference between push and not push? Do you use it /or find it more convienence then not using it? My phone is through verizon if that matters and i have the iphone 4s.

A "Push" message works on a signal sent from the phone company to the phone, which it gets immediately without having to do anything.
Then the phone operating system gives an alert or starts the appropriate app, to act on the message alert.

The alternate is "Polling", where the app has to be running in the background all the time and look for new messages etc. At regular intervals to see if anything is available.
That is slower and takes a lot more battery power.

The 1st answer was close, but actually push data is when the device is set to receive data automatically within certain approved apps. You ARE able to change the frequency that your device actively scans for data, but setting to "Push" relates to allowing your device to be constantly awake to immediately download new content (be it emails, news feeds, etc.) as soon as it is available. Push takes the most data and battery life but allows your device to be as current in content as possible.