Lines are multifaceted: by themselves they have a certain character in regards to their thickness, length, continuity, and shape. In the context of a photo, they can also align and connect objects, outline shapes, and seperate regions.
Generally, the lines you see in a photo are either real or implied.
horizon line, roads, railroad tracks, fences, trees, ledges, etc.
Objects that are edge or center aligned create an implied line
Similar objects can appear to be connected by straight or elliptical lines. An example would be three birds that appear to be the vertices of an implied triangle.
Regions of contrast (color or light) create an implied boundry line. The greater the contrast between the two sides of the boundry, the more prominent the line.
Another example of implied lines would be contour lines between the positive shapes (objects) and negative spaces (space between objects
The shape and direction of a line have a certain expresiveness and emotional impact.
denote stability, height, strength
example trees, people standing up, buildings, mountains
denote repose, stability, peacefulness
people laying down, flat lake, desert, or field
give the sensation of force, energy, motion, and action
trees bent by the wind, people in action
circles capture the eye, and keep the eye in the picture
ponds, outdoor track and fields
can offer beauty and charm
female form, river, path
charm, elasticity, grace, strength, and balance
roads, rivers, etc
The human eye is involuntarily drawn to junctures. Intersections are used as strategic points for placement in composition
Draws attention to the center of convergence
reinforce each other and add a strong sense of direction
Lines: what they do
Lines - Eye Movement
Lines - Tips