Depth of field - Dof
You've no doubtfully seen those 'professional' looking photos which blurry backgrounds. That's depth of field in action. Depth of field is the distance between the nearest and farthest objects in a scene that appear sharp in a photo.
A shallow depth of field will usually have a sharp, and in focus element, with a blurry background.
A deep depth of field will tend to have the majority of the scene in the photo in sharp focus.
There are a few factors that affect depth of field;
Lens length - A longer telephoto lens (85mm to 300mm for example) have an inherently shallower depth of field. Whereas a wider angle lens (12mm - 35mm) have deeper a depth of field, even if they are both shot at the same aperture.
Distance to the subject / subject distance to the background - Shooting at the minimum focus distance the lens permits will give a shallower depth of field and a blurrier background. Also, making sure the subject is a little distance from the background will create blurrier backgrounds.
Aperture - Generally a wider aperture (f1.4 for example) will help create a shallow depth of field. A smaller aperture (f16 for example) will give a deeper depth of field.
Combine all three purposefully to become a depth of field ninja!