Automatic: Usually represented by a green camera logo, the word "auto," or some combination of the two, this mode lets the camera decide everything about the exposure. Some cameras have a variant of the automatic mode, program (usually represented by a "P"), which enables you to dial in certain changes to sway the camera's exposure judgement, but either way the camera is making pretty much every decision about the exposure for you. This is by far the most used mode by the camera toting populous, but should it be? Well, that's really just for each camera owner to decide. I never use a camera in auto mode, mainly because I like to know what's going to happen with my shots ahead of time. Is there a time and place for this mode? Sure! If I've got a camera and I don't want to think about the pictures, I just want to take snapshots, I'll use auto mode. It is a very good mode for just quickly taking a picture without having to think about it. If you find yourself using this mode, I would however recommend you turn the flash off.
Shutter Priority: Usually represented by an "S" or "Tv" (Canon has to be different, means time value), this mode lets you dial in the shutter speed manually and the camera will determine the rest of the exposure based on this. This is one of the two "semi-automatic" modes, and is a great way to explore working with exposure without worrying too much about if your picture will turn out. The best uses for this would be when shooting fast moving subjects (sports, pets, kids), or when you are on a tripod and want to make long exposures to show/imply movement in the frame.
Aperture Priority: Usually represented by an "A" or "Av." This mode lets you select what aperture you want to use, then the camera does the rest. This is the second of the two semi-automatic modes, and I would venture a guess is the most popular shooting mode after automatic. I find myself shooting in this mode quite a lot. One of the benefits of a larger sensor and lenses with larger apertures is that you can turn backgrounds in photographs to gooey colorful butter. Using aperture priority mode lets me dial in that nice large aperture, and the camera takes care of the rest. When I use point and shoot cameras, I'll often use this mode instead of automatic so I can be sure I'm shooting at the widest aperture possible, so I gather as much light as I can, and therefore not have to use flash. The best uses of aperture priority would be when shooting fairly stationary objects, portraits, or landscapes.
Manual: Usually represented by an "M." This mode gives you total control over the aspects of exposure. This mode can be scary to a lot of people, mainly because there seems to be a learning curve involved in using it. In reality, this mode is all about information. Instead of setting your aperture and hoping the shutter speed or ISO that gets automatically chosen will work, you know it will or won't. It does require a working knowledge of the exposure triangle, but this mode (unlike the automatic or semi-automatic) makes it much easier to visualize what's going to happen since you are controlling everything. The downside? It takes a little more time to use. You won't be able to go from one lighting zone to another and just press the shutter button right away like the other modes- in manual you will have to change the settings. I only recommend this mode if your camera has the dials or buttons to make it convenient. Point and shoot cameras are simply not set up to handle this- and it's one of the reasons why you see so many buttons and dials on dSLRs. If you plan on doing any sort of on or off camera lighting with something other than the built in flash, this mode is essential.
Other Modes: These would be all the other possible modes that each camera offers. Since nearly every camera offers the basic four modes I just listed, the "other modes" can be a reason to pick one camera over another. The Canon G12 offers lots of modes such as "color accent" which makes the entire image black and white except for the selected color and "nostalgic" which lets you select 5 different photo aging effects to place on the picture. My wife's Olympus E-pl1 has a mode called "Art Filters" which lets you apply an effect to pictures taken such as soft focus, grainy film, and pin hole camera- and shows the effect on the screen while you compose! Other common modes are things like HDR and panorama, which allow you to create images in camera that used to take special software on a computer to accomplish. Most of these modes are automatic, but so much fun!
Here's a bonus: 5 quick tips regarding shooting modes!
- Use the on camera flash in emergency situations only. Even if your camera wants to turn it on, override it. I'll go into detail in an upcoming tips post.
- Some cameras enable you to lock in exposure variables in the auto and semi-auto modes, giving you a sort of quasi-manual mode. This is a great way to "test the waters" with manual exposures.
- Some cameras enable you to set maximum/minimum exposure variables for the camera. The most common is maximum ISO, but some cameras will also allow you to set shutter speed. Using this feature can help you have some sway over how the camera exposes in auto.
- The main reason to upgrade to a dSLR is control over your images. It isn't a magic device that makes all your pictures come out better, rather, it lets you fine tune aspects of your image that is difficult or impossible to do on most point and shoots. I've taken plenty of crappy shots with my dSLR.
- Point and shoot cameras usually offer more shooting modes and features than dSLRs. If you really like exploring different shooting modes, they are introduced in the P&S cameras first, and might eventually wind up in dSLRs.
Finally, a couple links regarding shooting modes: