It's neither; it's a gerund (colloquially, an "-ing word"), a form of a verb (in this case "provide") used as a noun. Here it's part of the noun phrase "providing fans with wireless Internet service or Web access at their seats," the activity in which the Giants are dabbling.
Verbs conjugated with "-ing" can also be used as adjectives (they're called "gerundives" when used like this), as in "caring parents" or "loving wife", but using "providing" in this way doesn't sound right to my ear, and it's definitely not what's happening in the example you gave.
As a side note, "provided" can be used as a conjunction; it's a synonym for "if":
You can go to the party, provided you're home by 12.