Composition is the over all layout of an image and how it appeals to the viewer.
A well composed image takes into consideration such points including the rule of thirds, lighting, subject and contrast. A perfect example is a sunset. A well composed image will either have 2/3rds sky with 1/3rd water or vice versa. A poorly composed image puts the horizon line right in the middle.
Something else to consider when composing an image are leading lines and objects in the image. Are there distractions within the image? All to many times we have seen the classic photograph of a person standing in front of a telephone pole off in the distance. This gives the wonderful effect of a telephone pole growing out of somebody’s head. Another example is a photo of a barn yard animal with a fence behind it to give the appearance of the fence growing from its head.
Notice below LEFT, although lighting may be suffice, and the image is pretty clear, there is a pole right in front of the animal. The animal, also compared to the size of the image frame is much to small with to many distracting lines in the image, giving the image the appearance of a “snapshot” type image. The image on the RIGHT has fantastic composition. The look in the cats eyes is intriguing as it is not looking at the camera straight on. The cat also fills the entire frame of the image, not just a small percent of the frame.