whenever people talk composition they always think "rule of thirds". let's break our subject down to the basics, lines. lines suggest or lead. we can use this to our advantage when we create images.
1. vertical line - masculine or a line of strength. in the early days they always had the gentleman stand, sometimes next to a grecian pillar and add a book to imply wisdom.
2. horizontal line - line at rest or pastoral. okay for landscapes but can become boring if you are not careful. since we speak English, we read from left to right, so a group of people standing in the fig leaf position could become a linear row of heads.
3. diagonal - line of of action. you will follow a diagonal line. let's take a guy just standing with his hands straight down, a vertical line. now let's have him place his hands in his pockets with the thumbs out. now his arms are bent at the elbows creating diagonal lines. now let's bring over his buddies. have one sit on something in front and another stand next to our model. now we have created a triangle with the heads. your eye will move around the the which is now more dynamic than the original row of heads.
4. "S" Curve - the feminine line of nature. yes you could pose a person in this by having one foot presented forward and bending the knee in towards the other. this also shifts the shoulders the opposite direction. mostly we see s-curves in calendar type shots as winding roads or creeks. this is a graceful line that your eye follows slowly. a waterfall would be an example. using a long shutter speed blurring the water even better.
5. zig zag - line of conflict. you're not going to see many of these on office calendars. accelerated line of action creates excitement.
6. converging lines - always have a vanishing point, whether you can see it or not. your eye will seek it out even if it's not there.

okay quiz time:
you go to a waterfall, you set the shot up perfectly using the rule of thirds and decide to take a shot with a person in it. the person gives a scale of reference to the waterfall.
What's the difference between a snapshot and a postcard?
if the person looks at the camera, it's a snapshot. you will look at the person.
if the person looks at the waterfall, it's a postcard because you will look where they looking.