The Four Main Modes in DSLR

Before we see the four main modes, let’s have some basic understanding on the scene modes.

Scene modes such as Landscape, Night, Macro, Portraits, Sports, Sunset and so on are the modes preprogrammed by the manufacturer to automatically give the best exposure and settings for each scene. Sometimes we call it Auto Modes because we just need to select the scene and let the camera adjust the setting combination automatically.
To read more about Scene Modes, please click here.

PASM Modes
The four main modes, sometimes abbreviated "PASM", are Program, Aperture Priority, Shutter Priority and Manual. Normally you can found it in the manual guide when you purchase a new DSLR.

So what are the differences?
P: Program mode has the camera calculate both shutter speed and aperture (given a manually or automatically selected ISO). The difference between Program mode and Full Auto mode is that in program mode, only the exposure is automatic, while other camera settings (e.g., shooting mode, exposure compensation, flash) can be set manually; in Full Auto mode everything is automatic.

A or Av: Aperture priority or 'Aperture value' enables manual control of the aperture, and shutter speed is calculated by the camera for proper exposure (given an ISO sensitivity).

S or Tv: Shutter priority or 'Time value' enables manual control of the shutter speed, and aperture is calculated by the camera for proper exposure (given an ISO sensitivity).

M: Manual mode both shutter speed and aperture and independently set manually (with ISO sensitivity also set manually), where proper image exposure requires accurate manual adjustment.

There are some less commonly seen modes like:
- Sv (Sensitivity Priority or ISO Priority)
- DEP (DEPth of Field Mode)
- A-DEP (Automatic DEPth of Field Mode)

These modes are only available with certain brands and models.
*Source from Wikipedia.org