Rule of Thirds
Imagine a blank piece of paper, now divide the paper equally with two lines vertically and two horizontal, what you can now see is a basic grid used by photographers all over the word, it is called the rule of thirds. The rule of thirds was developed by landscape artists who used this technique to keep focal points out of the centre of a picture and put them instead either a third of the way up / down or in from the left / right. It simply makes most pictures more interesting to look at.
Photographers were quick to pick up on the idea and is now used so much that many manufactures include some sort of a grid system for their cameras. If possible the main focal point of interest in the picture should be captured where your grid mines meet, these are known as power points. As with all rules they are made to be broken so do not miss a shot because it doesn't fit but if you are not already try it out!