Aperture-the opening that allows light to hit the camera’s sensor. 

1. Aperture values are expressed in numbers called f-stops. A smaller f-stop number like f2.8 or f5.6 means more light is coming into the camera. A larger f-stop number like f16 or f22 means that less light is coming into the camera. Adding a full stop of light doubles the amount of light reaching the sensor. 
2. The full stops for aperture are: F2, f28, f4, f5.6, f8, f11, f16, f22, f32

 

Aperture is a major factor in determining depth of field.

Depth of field-the distance between the nearest and farthest points that appear in acceptably sharp focus in a photograph. Depth of field can be extensive or shallow.

See an example of extensive depth of field: Bill Brandt, East Essex Coast, 1957
Everything from the ear in the foreground to the cliffs in the background is sharp.

See an example of shallow depth of field: Roy DeCarava, Picket Demonstration, 1963
Only the man in the middle distance is in focus. the person before him and the person behind him are not in focus.

 Depth of Field relates to focus. It is not to be confused with Perspective-the representation of a 3-dimensional space on a 2-dimensional surface by converging lines, diminishing scale and/or atmospheric perspective.

Control DOF with: lens aperture, focal length, camera-to-subject distance, sensor size.

 

 When photographing for shallow depth of field:

• Use Aperture Priority or Av and set the camera to the widest aperture (lowest number.)
• Use the telephoto end of your lens (zoom in.) 
• Get close to the subject and allow some distance between the subject and the background. There must be space between the foreground and background of your composition. 

 Bokeh-Bokeh comes from the Japanese word boke (ボケ), which means “blur” or “haze”, or boke-aji, the “blur quality.” Bokeh is pronounced BOH-Kə or BOH-kay.  — From http://www.nikonusa.com/en/learn-and-explore/article/h0ndz86v/bokeh-for-beginners.html