The most important examples of topoi are categories of sheaves of sets on a small category. Patrick Eilliott introduced this class of examples over two talks, of which is the second. In this talk he defines Grothendieck topologies and the category of sheaves on a site, and develops the example of the Zariski topos. This example will return later as the classifying topos of the theory of local rings.
The lecture notes are available here: therisingsea.org/notes/ch2018-lecture7.pdf.
For the general seminar webpage see therisingsea.org/post/seminar-ch/.