Now, if you're not familiar with the Real ID system, I'll give you a quick summation. If you have a real-life person you know who also plays Blizzard games and you would like to keep in touch with them despite realm, faction or even game (Diablo III, StarCraft II or World of Warcraft), then you can now add them in a social media-type format with announcement, statuses and cross-realm/faction/game whispers. This is not something you want to do with someone you've only ever known in game. The reason you don't want to do this with just anybody is that you will be using your Battle.net email login.
If you happen to try out this feature on the PTR, please note that we're still in an early test realm build and there are still a lot of bugs in the system. This means you're going to get Lua errors off of the default interface, and the Battle.net server managing your friends list is often down as they're tweaking things behind the scenes. It's the test realm; it's meant to test things. So don't take its stability as a sign of what the functionality will be like when the patch hits the live realms.