Focal Length
Focal Length is about a lens's viewing perspective and is expressed in millimeters (mm).
I photographed these examples several years ago, using 2 different cameras and multiple lenses to cover the entire focal range seen here.
Watch the tombstone in the center and notice how our perspective changes from one photograph to another.

18mm
Normal human viewing perspective has always been considered to be roughly 50mm.
Although some would argue that technically with digital cameras it's really between 38mm and 43mm, I'm still going with 50mm.
Anything lower than 50mm pushes objects away from us, opening up our field of vision (wide-angle and fisheye).
While going over 50mm brings objects closer to us (telephoto and telescopes).