It depends on where you live.

In most urban and suburban places, it's illegal. There are laws detailing which body parts constitute "indecent exposure," including any visible form of sexual arousal while covered. Typically, violators are arrested, charged with a misdemeanor, and fined heavily. The main offenders tend to be mentally ill people and those who are intoxicated or high.

In most other areas, it's an issue subject to complaints. It's kind of like running stop signs on a country road at 2:00 A.M. - if no one saw you, it didn't happen. If they did, and complained, you might get a visit and a warning. Get multiple complaints, and the local busybodies will get a law passed.

Outside of private resorts, nudity is only legal (meaning there are no specific laws prohibiting it) in a few places in the USA, so long as there's no overtly sexual behavior accompanying it. (Sunbathing nude in your backyard? Not a problem. Helicoptering on your balcony? Jail time.) There are a few exceptions: participants in public protests like the World Naked Bike Ride, and the Folsom Street Fair are granted temporary immunity from arrest if they remain within a designated space.

There are a few activist groups working to promote the virtues of body acceptance and nude recreation. Both are gaining popularity, as more private clothing-optional communities are being built, others are making enough money to renovate, and clothing-optional vacation destinations and cruises are becoming more popular.

Culturally, there's a rift between those who (very generally speaking, here) uphold nudity as non-sexual, and those whose activities are more sexually oriented (i.e. swingers, honeymoon resorts, etc.).