It doesn’t get much simpler than these (simmer down, Foster-Harris, you’ve had your turn):
In the first scenario, some game-changing event occurs when the unknown invades the known. Perhaps a mysterious new girl shows up at school, or a long-lost uncle knocks on your door, or aliens invade Earth, or a posse of cowboys sidles into the local tavern. Whatever the unknown factor is, it’s found its way into an established setting and has proceeded to muck about with the status quo.
In the second scenario, plot occurs because the known is taking its show on the road to locate and get its hands dirty in the unknown. Here, maybe the main character is the new guy (or girl) in school, or he hitches a ride on the last bus to Vegas, or he is part of the invading party on a new world, or he decides to buy a theme park and renovate it. Whatever the known factor was doing, they/it/he/she has laid the known aside in pursuit of the unknown.
When pondering these two types of plot, it’s easy to think of them as commenting only on characters. A character leaves or comes to town. But no, these plots may also refer to other known and unknown elements in the story. Consider, for instance, a hurricane blowing through southern Florida or a new Wal-Mart being erected in a small town. Those are both examples of “a stranger comes to town” (unknown to known). How about a character’s house getting repossessed or the world plunged suddenly into a zombie apocalypse? Those are examples of “you leave town” (known to unknown). Keep in mind that not every plot element need arise from the actions of characters; sometimes their worlds act on themselves and create change.
Another fun note: many writers have pointed out that these two types of plot are really the same plot viewed from two different perspectives. Mind blown, right? Ours too.
Why is this important? Well, like Foster-Harris’s assertion that conflict is the only plot worth having, these two plot types suggest that change creates plot. If something doesn’t change, then there’s no story worth reading. So, change and conflict may be considered complementary here, or even the same thing.
Regardless, when you are developing your plot, look for ways to incite change to create conflict. The only two ways to do this is to either thrust the unknown upon the known, or to pull the known into the unknown. Choose your poison and drink it down.