The "+" means it's one of their "Plus" shoes and compatible with the Nike Plus / Apple iPod system they have available. Really any shoe is "compatible," but the Plus means there's a pocket in the sole of the shoe in which the sensor can be placed so you don't have to attach it to the outside of the shoe.
The 7 means it's the seventh iteration/design of that particular model. They update shoe models from one year to the next, usually with some tweaking and other alterations, but newer by no means automatically means better.
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