HyperText Transfer Protocol (HTTP) is the protocol for moving information across the Internet. All webpage addresses begin with "http://" or "https://" (for a secure connection) but most new browsers hide this from you. We say Ignition uses HTTP to communicate between the Clients and the Gateway. This just means that Ignition is using an existing, proven technology to transfer its information throughout the network.
Historical Data Access is a specification for exchanging archived, or historical, process data.