What does it mean when it says free app with a plus sign?

I just got my new iPhone 4 and I want to download a free app but I don't exactly know what that means or if it'll cost me later.

The plus sign indicates that the app is available for the iPhone and available in HD for the iPad.

http://ipod.about.com/od/iphonehowtos/qt/Turn-Off-In-App-Purchases-Iphone.htm

Plus sign indicates its an universal app which is compatible for both iPhone and iPad.
Incase you download the app which is free it will never cost you later even if the app goes as paid.
Few apps has got In App purchase which will cost you incase you wish to download it.

Hope this helps you.