What is Focus?

Focus is a point at which rays of light converge. In photographic terms - focus isn't typically a point but an entire plane in front of and parallel to the camera's sensor. 

How is focus determined?

• Your camera has a focus sensor. This sensor uses various elements and techniques to detect when a focal plane is achieved - that is - there is at least one plane in your composition from which the light is properly converging on the sensor. 
• Once achieved, Focus is only a factor of the distance of the subject from sensor of your camera
• Auto-focus is switched on (or off) via a small button on your lens

Focal Length

• The focal length of the lens is the distance between the lens and the image sensor when the subject is in focus, usually stated in millimeters (e.g., 28 mm, 50 mm, or 100 mm)
• Focal length of 50mm (35mm for cropped-sensor) is considered "Normal"
• Focal length of  less than 50mm is considered "wide-angle" and larger than 50 are "telephoto" 

Auto-Focus

• Auto-focus works off of contrast in a scene. If you are trying to take a portrait then focus on the eyes not cheeks.
• Key component of auto-focus is focus points. These are locations offered by your camera at which the camera needs to auto-focus. 
• You activate auto-focus by half-pressing the shutter release button. 

Manual Focul-point Selection

• Cameras typically give you a grid of points at which focus can be achieved
• These points are activated automatically based on what the camera's auto-focus determines are spots with best focus. 
• Many times, what can be focused the best isn't necessarily where you want to focus. 
• Most cameras allow you to select a specific focus point manually using some manual or digital control. 

Focusing Tips

• Use Auto-focus as much as possible
• Focus on the eyes of your subject.  
• For more interesting compositions keep the focus off-center. 
• Since focus is only about distance and not about the specific point, you can focus on one thing and keep the shutter release pressed halfway and recompose without losing focus.
• For truly interesting photos - you need to learn to select focus points manually and not rely on your camera's focus point selection