What is DOF? The distance between the nearest and farthest object.

Whats the deferent between a small DOF from a large DOF? A small DOF show only what your focussing on and give you a blurry background but a large DOF show both back and front not really focussing on anything but everything.

How those your photo show DOF? My photo show a small DOF because i'm only focusing on one thing.