Chocks are most often found on the bow of a boat to act as leads for lines, or even the anchor rode. you lead lines through the chock to help reduce chafe on the line (for example, the lines used to tie up a boat in a slip), and possibly to give the line a clear run to the clete itself.
But chocks also can be found on the stern, or even the sides in the toe rail, of some bigger boats--wherever there''s a need to produce a "fair" lead for a line, and to protect it from chafe.
One suggestion is to get a BOAT/US or West Marine catalog. Those often have photos of various gear in use, or at least situated on a boat. And if you can, also meander through a marina looking at boats and asking questions of folks you see on them. I have yet to meet a sailor who won''t talk about his boat no matter what the question is. Good luck.