Yahtzee wrote...
...basically someone takes it upon themselves to dissemble the entire course of a game into screenshots and videos, annotated with their own commentary. Some LPs are there to highlight an underappreciated gem, some to tear apart a big fat turd, but a well-conducted one is always a criminal timesink for me.
And that's pretty much what it is. There's a website full of them
HERE.
As an example, here's an awesome one, done on Resident Evil 4 by
The Dark Id:
http://lparchive.org/LetsPlay/RE4/