If you see a + sign next to or below an app in the iTunes App Store, it indicates that app is universal -- designed to work natively on both the iPhone and iPad.
Typically these universal apps are provided by developers when the functionality is similar on both iPad and iPhone, but due to the iPad's large screen a different interface is needed (to include sidebars, popovers, and wide views instead of deep levels).
It also means you only have to download -- and pay for -- the app once.
Sure, you can run any iPhone app on the iPad if you're okay with boxing or chunky double fuzzy 2x mode, but universal apps ensure you get the best user interface -- and user experience -- possible on all your current iOS devices.
So if you have an iPhone or iPod touch and an iPad, and you see the + sign for a universal app, you're good to go.