Composition

Placing the elements of your picture within the frame and deciding what to leave out.

In our modern world of automatic cameras, which focus for us and adjust the exposure in an ever more perfect way (most of the time), the biggest difference between a good photograph and a mediocre one is the composition. In every photograph we take, we can decide where the boundaries of that photo will be, called the cropping. We can also choose the viewpoint. If we are taking pictures of people or movable objects then, often, we also have the opportunity to arrange them into the shapes we want.
I use the word 'shapes' deliberately as that is what good photography is all about, creating shapes and textures that please you.
Landscapes If you are shooting landscapes or other immovable objects then you must compose the picture by moving yourself and deciding where to place the point(s) of interest in your picture.
There are various compositional rules (I prefer to think of them as guidelines) to help you. These rules will help you to compose pleasing pictures, however, you will often find that a really striking picture will show a blatant disregard for the rules. Once you are aware of the rules then break them as often as you want but, at least, know you are breaking them and why.
One important rule to take a look at is the very popular Rule of Thirds

Using Diagonals

Setting your subject matter on a diagonal will almost always make for a more dynamic picture. Even if this is an invisible diagonal that draws your eye between two points. Move around the subject (not too close in the case of my crocodiles) and look for a diagonal.

The art of good composition is first of all choosing the right viewpoint, then placing the main elements where you want them in the frame and finally making sure that the cropping is to your liking, make sure that there is nothing untidy at the edge of the picture.

For some inspirational help with composition try Learning to see Creatively