A mode dial or camera dial is a dial used on electronic cameras to modify the camera's mode. Most digicams, including DSLR and SLR-like cameras, support modes, selectable either by a dial or from a menu. On point-and-shoot cameras which support modes a variety of scene types is offered. On DSLR cameras and SLR-like cameras, mode dials typically offer access to manual settings. The more cameras and compact point-and-shoot cameras offering a great many modes don't have mode dials, using menus instead. Some SLR lenses themselves offer control of things such as aperture, reducing the requirement for mode support in the camera body.

Auto
Sometimes this is indicated by the word Auto occasionally a green square. What does it do? In this mode the camera will decide everything for you, the aperture and shutter speed, the ISO, whether to use the flash and so on.

P – Program
Routinely it is denoted by P. In this mode you're still using the camera’s car exposure calculations but you've more capability to override the camera choice. You can set the ISO yourself, decide on the proper metering mode, and select when to use the flash. You don't immediately set the aperture and shutter speed, however if you're presented with anything except average conditions you can indirectly control the settings using exposure compensation or P shift mode.

A – Aperture Priority
Here you have all the same control as P mode with one key additional setting to be controlled that is the aperture value. You set the aperture based mostly on the sort of shot you would like, shallow DoF then set a low number, landscapes then crank it up.

S – Shutter Priority
This is sort of a mode but rather than setting the aperture by hand you set the shutter speed. When you want to guarantee the shutter speed is a certain speed to freeze motion, blur motion, stop camera shake etc you may use this mode. The camera will select the aperture and ISO.

M – Manual mode
Here it is down to you. You want to set everything to get a properly exposed photograph. Aperture, Shutter Speed, ISO, the entire triangle! Some individuals believe in it and never shoot in anything apart from manual; some folks are terrified of it. Manual mode has its uses, but sometimes it just not that required.

Scene Modes
Masses of modern cameras have a mess of scene modes, meant to help you get the right shot in some situations.

Landscape mode
Landscape mode favours smaller apertures to maximize DoF. JPEG conversion favours green and blues and boosts saturation. I can also switch WB to light.

Portrait Mode
Portrait mode favours bigger apertures to make nice fuzzy backgrounds. JPEG conversion favours skin tones.
Macro Mode
Macro mode is absolutely different in a DSLR to a Point and Shoot camera. In a DSLR it will favour medium apertures and clear JPEGs. In a Point and Shoot camera it will permit closer targeting and create (a touch) more hazy backgrounds.

Sports Mode
Favours quicker shutter speeds to freeze motion and turns on constant shooting mode.