For the sake of argument, the Real World may be understood as the one that contains the server that hosts this wiki. Usually, people referring to the Real World have the planet Earth in mind, but they may also be referring to the entire space-time continuum in which Earth resides. It is where your Real Life takes place.
The inhabitants of this world either don't know about the PPC or are aware that the whole multiverse that contains Headquarters and its agents is the product of collaborative writing and just a tiny corner of the Internet. It is where the people who chronicle those agents eat their tea.
Though the Real World always contains Earth, stories about Earth are not always set in the Real World. In fact, strictly speaking, the only stories truly set in the Real World are what we might call "history," and even there there's room for argument about what really happened. But let's not worry about that here.
"Alternate Earth" is the setting for many a canon, including fantasy continua like Harry Potter, Pirates of the Caribbean, and Buffy the Vampire Slayer; realistic continua like House, M.D. and NCIS; and sci-fi continua like Star Trek, Stargate, and Farscape, which don't limit themselves to the planet Earth, but take place almost entirely elsewhere in the galaxy, sometimes even elsewhere in the universe.
In the PPC
World One is the name of the alternate universe that contains an Earth virtually identical to the Real World's Earth except that the PPC can be accessed from it by plotholes, portals, and Doors into Headquarters. All agents who claim to be from the Real World's Earth are actually from World One or some other virtually identical AU. Most of them do not know this, and we don't recommend telling them. They're insane enough as it is.
World One is (usually) the world by which the HQ Standard Timeline is determined.