a lil writeup from me to understand exposure with analogy to filling a bucket of water..
FarBird Photoblog: ISO, APERTURE, SHUTTER SPEEDS and EXPOSURE
I've learned abit here & there, I spent a lot of time post-processing thru LR + CS. But I'm trying to snap good photos now. I really need to learn these things... Maybe I'm like learning how to run before learning how to walk!
A mode for nite shots if you have full-frame equivalent F2 or faster lens , or for shallow (or deep) Depth of Field
S mode for sports, shooting action, stopping action, pan, long exposure.
P mode for learning on the dial. turn the main dial around to see what adjusting aperture does to the shutter time, etc . Take a photo and use M mode to go further.
M mode for developing from P mode - you can adjust the exposure by setting the desired Aperture value seen in the previous shot, then adjusting shutter time longer (brighter image) or shortening shutter time (darker)
Read your manual.
Actually... there is no real golden rule to say which mode you should use for what type of photography. Although what others had already explained in previous comments, it was what people normally use. You just need to know what each mode was, Aperture Priority (set your aperture manually, while camera choose your shutter speed for you), Shutter Speed Priority (set your shutter speed manually while camera choose your aperture for you), Manual mode (set your shutter speed and aperture yourself). Auto-ISO - camera set your ISO for you automatically depending on the environment.
It is the same when people say you need a small aperture for landscape shots... while it I true in most sense... but sometime you really don't need to... depending on the distance your subject was away from you... if you focus to infinity, then even at maximum aperture of say... f4, your subject will still be in focus... I hope what I say here made sense.
Anyway, you would need more experimentation and read up more... then join some outings.
Everyone has their style of shooting.
I personally don't find Program (P) mode very useful, but the name itself is pretty much self explanatory. It's basically Auto+ for me. Nonetheless it has its uses for sure..
Keeping in mind the 3 parameters in photography:
1) ISO - this affects amount of noise and also the speed of your sensor in capturing light.
2) Aperture - this affects the depth of field (i.e. how much of the picture is in focus) as well as how much light your lens can capture during a given set of time.
3) Shutter speed - this affects the time the shutter is open for and the amount of motion in your picture.
The end goal in a photograph, when it comes to technicalities, is exposure. There is no right exposure but to get a certain exposure you will have many sets of ISO/aperture/shutter speed combinations. They all exert a relationship on each other. For example when you need to reduce the shutter speed while retaining the depth of field in a picture (perhaps because you can't get a good handheld sharp photo with existing shutter speed), you then keep aperture, reduce shutter speed and dial up ISO. So on and so forth. There's a wealth of information on this, go read up if you don't already know. Now with THAT in mind...
M/Manual mode offers the most control as you can exercise discretion over every single setting. Useful when you need to fix the exposure, for example, if you're taking a set of images to stitch iThe downside is that you have to fiddle with more things. Not very useful for situations where you need to change the settings fast with less thinking, e.g. when shooting in constantly varying lighting conditions. In general, good for landscape photography.
A/Aperture priority mode prioritizes the aperture. So your concern here is the amount of depth of field. The camera then selects the aperture speed for you, and you may or may not fill in the blanks for ISO speed... Useful for situations where your priority is shallow DOF for sure, e.g. portrait photography. Downside, less control. You're releasing more and more control to your camera meter, which can be fooled at times.
S/Shutter speed priority prioritizes shutter speed. So your concern here is the amount of time the shutter is open for. The camera selects the aperture for you, and you may or may not fill in the blanks for ISO speed... Useful for situations where you need to capture something FAST, or if you don't think you can handhold above certain shutter speeds, e.g. sports photography. Downside is also less control.
The above are just generalities and everyone's style is different. I know wedding photographers who prefer to use manual mode all the time despite the fact that they're in a fast paced environment with very tricky lighting situations, because these situations fool the camera meter too! I also know landscape photographers who use aperture priority because well, they just prefer it.
What should you use? Just try la. Then you can answer the question yourself.
You should read up on these articles...
Diffraction: When Smaller Apertures No Longer Mean Sharper Pictures - Photo Tips @ Earthbound Light
Understanding Lens Diffraction