DSLR stands for Digital Single Reflex Lens.
DSLR’s took over SLR’s in the 2000s because of its use of a digital sensor to detect light rather than film. this means that they were a lot cheaper and had better image quality.
A DSLR view finder will normally use a mirror to look down the lens which will fold up as the image is taken as seen in the video below.This is where reflex fits into the name.
the ISO determines how sensitive a camera sensor is to light. the most common settings are 100, 200, 400, 800. the lower the ISO the slower the speed and the less noise you will have effecting the image. for example if you took a picture or film of a sign or poster at 100 ISO the color and text would be clear, but if you took the same picture at 1600 ISO there would be noise distortion effecting the color and text which could mean it can’t be read. However you can’t always use 100 ISO as this shot needs plenty of light and little movement so for many shots you would have to experiment by starting high and getting lower and reaching a compromise.
Aperture is like the pupil of an eye. it is the opening through which light travels. this controls the depth of field of the shot so how much of the image is in focus. if the hole is wide more of the shot will be in focus but if it is smaller the point that is in focus is much finer and so the eye is drawn to a specific point.
Shutter Speed is the speed at which the shutter opens and closes thus changing the amount of time the sensor is exposed to light. the faster the shutter speed the crisper the image but the darker it is also. a fast shutter needs more light and a slow shutter needs less.