We know what Kellogg is. We live here. You don't need to explain it to us any more than you need to explain to a Philadelphian what Wharton is.
It might help to clarify what you mean by "pop-up." They can take on a few distinct forms, such as:
a) a temporary alternative location for an existing restaurant that is well-publicized in advance, possibly looking to test the market in said temporary location;
b) a temporary location, also typically well-publicized in advance, for a well-known chef who may be in between assignments and/or looking to establish his/her own restaurant;
c) a temporary collaboration between well-known chefs and/or restaurants sometimes hosted at a third-party site; or
d) "Fight Club" pop-ups by would-be up-and-comers that are secretive, invitation-only affairs and fly under the radar not just of the general public but also of various government tax/inspection/regulatory institutions. These are the culinary equivalent of would-be rappers trying to build buzz with mixtapes and word-of-mouth.
Which one(s) of the above are you interested in?