APERTURE VALUE - Definition

Aperture value is the size of the hole the light passes through.

Small Opening - Large Aperture Value
Large Opening - Small Aperture Value

Aperture is found in your camera's lens, not in the camera!

Lens Specification:
85mm 1:1.2 :  Here Milli meter  is the focal length and 1.2 is the widest aperture value of the lens. '1:' means 'in a minute'. The Folca Length always comes first followed by the Aperture Value.
70-200mm 2.8:  If the focal length is a range, it means that the lens is a zoom lens. It zooms from 70mm to 200mm.
70-300mm 4.5-5.6: Obviously its a zoom lens which zooms from 70 mm to 300mm.  Now comes '4.5 - 5.6'. This means the aperture will not be stay constant while zooming. It varies. Most of the consumer lenses are of this category as lenses which give constant aperture value, costs more.

WHITE BALANCE

To solve issues with color.

The light source may be warmer or cooler. Adjust the white balance first and then take a picture.

DEPTH OF FIELD

Shallow depth of field. : It means subject is in focus and background is out of focus. 

There are THREE things that control DEPTH OF FILED. 

1. Aperture 
2. Focal Length
3. Distance from Subject.

1. Aperture: To get a shallow depth of field, you need a wide (large) aperture opening (a small aperture value). The larger the opening, the shallower the depth of filed.  

2. Focal Length: Focal length also have a great impact on depth of field. When focal length increases,  the depth of field decreases. 

3. Distance from the subject:  The closer you are to your subject, the shallower your depth of field will be.