TITLE: "Every scheme as a sheaf" references?
QUESTION [16 upvotes]: I have sometimes hard time reading papers that are written in the language of schemes being replaced by the functors they represent (I have especially homotopy scheme theory in mind). 
I think the topic is connected to topoi and Grothendieck topologies, but for now I'm looking for something simple, just the working overview of the language of representable functors, what is a scheme, etc.

REPLY [3 votes]: You may also find the notes "Introduction to Functorial Algebraic Geometry" useful.  They are based on a course given by Groethendieck - and can be found here.  Unfortunately they're a little rough around the edges sometimes and slightly dizzying to read because of the scanning.