I've seen a couple other questions such as this one talk about focusing screens. What is a focusing screen? Where is it located?
On a reflex camera (those with a mirror allowing you to compose and focus through the same lens that you will shoot the picture - aka SLR or DSLR) the focussing screen is a glass surface on which the image is projected by the mirror. You can see it by removing the lens and looking inside the body above the mirror:
When looking into the viewfinder you see the focussing glass through the prism which inverts the image (the image on the focussing screen is flipped horizontally).
The focussing screen may be engraved with design such as the focus points on a DSLR, however on high end cameras it is possible to replace the focussing screen with one displaying a grid allowing you to better compose the image or an other such design.
For manual focus purpose some focussing screens have a micro-prism in the centre which splits the image when out of focus.