Happy to answer that for you. Roaming is the general term for anytime your mobile device is communicating with a network other than Verizon Wireless. Data refers to the data signal that is used to transmit data such as email and web. USA refers to roaming inside the United States as opposed to Global which is billed differently. "Data Roaming USA" is domestic communication with a data network other than VZW. Most calling plans include it.
I might be able to provide further insight if you could tell us why you are asking. Is this a feature you see on your line, something listed on your bill or something you see on the phone?