Realms: It's the best guess about the status of a given realm within a region. If the ping results are too high for 1 or more hosts, or we have problems connecting to the game ports for multiple servers, we mark all realms within a group as having problems. The only time an individual realm is marked as degraded or offline is when FFXIV reports it as offline.
Datacenter: each datacenter has a single server that handles login (once you're in the game client), realm selection, displaying your characters on a realm, and creating new characters. We highlight the name of the datacenter with the guessed status of that lobby server.
Global Services: We display status for the Login and Gate servers. Login is where you type in your username and password. We report if this page loads successfully. The gate server isn't actually a server in the flow of logging in, rather it shows general status that FFXIV reports to the game client. If the "gate server" is down, the FFXIV client prevents people from logging in.
Ping times shown here are the round-trip time from is.xivup.com to each of the FFXIV game servers. If any of the pings are over 200ms, it's likely there are issues with the internet between this server and the game servers. This can help show potential problems with the game servers.