Types of exposure in cameras
SRL and DSRL cameras have different types of exposure. Compact cameras sometimes only have 1, generally automatic, and some newer ones have several as well.
1) Aperture priority (AV or A): when we use this mode of exposure, we set the diaphragm we want to use on the camera and the camera sets the necessary shutter speed for a correct exposure. This mode of exposure is used by many photographers when they want to have control over depth of field since they choose the diaphragm to be used. It is, to some extent, automatic.
2) Shutter speed priority (TV or S): when we use this mode of exposure, we set the shutter speed we want to use on the camera and the camera sets the necessary diaphragm for a correct exposure. We use this mode of exposure to control movement in a photograph. This is also a semi-automatic mode.
3) Automatic (A) or Program (P): The camera measures the light and sets both the shutter speed and the diaphragm. In other words, it chooses the correct combination. We can't control movement nor depth of field. This mode actually limits the creativity of photographers.
4) Manual (M): this is the mode I personally like to use. As a matter of fact, I use it all the time. When we set the camera in manual mode, we decide both the diaphragm and shutter speed we will use for a specific photograph. By doing this, we are free to allow more or less light into the camera than the quantity dictated by the photometer to compensate or simply to change a photograph. This is actually the most accurate mode we can use. Professional photographers use this setting. However, using manual mode requires more time; we need to measure the scene correctly to make sure we get the right exposure. When we are taking pictures using high shutter speeds (sports, news, etc.), it might not be the most convenient mode to use. At least, not at first.
As always, I recommend (and I don't won't stop saying this) lots of practice, practice, and more practice. This will ensure we understand how the camera, lenses ISO, the diaphragm, the shutter speed, the photometer and exposure work. We need to take countless photographs. It is the only way to improve the quality of the images we produce.
Try taking pictures using all diaphragm settings, every shutter speed available to your camera, using different film speeds. Try different light situations; very bright scenes and darks scenes too, indoors and outdoors. Try shooting still objects and moving objects. You now know what I'm talking about.
Finally, observe your photographs carefully and analyse which came out correctly and which ones didn't. You will get excellent results and not so great ones as well. This way, you can avoid future mistakes and repeat things you did well.
