Quest for Booty is not your standard Ratchet & Clank game. A lot of terms have been thrown around to try and explain the Quest for Booty: expansion, add-on, episodic, etc. According to Insomniac none these terms is entirely accurate. For one the game is not an add-on or an expansion in the typical sense. Yes, the game does continues the story of Tools of Destruction but it is also completely stand-alone, so you will not have to have the original in order to play Quest for Booty (like some initial reports have stated).
It is also not episodic in the typical sense. Quest for Booty is certainly an episode in the adventures of Ratchet & Clank but at this point no additional episodes have been announced. Instead, Insomniac is looking at Quest for Booty as something new. The best way to describe it is a smaller game at a cheaper price with the same high quality production values and excellent gameplay you have come to expect from the Ratchet & Clank series.