SLR = Single Lens Reflex
(What you see in the viewfinder is reflected off a mirror. So what you see is the actual picture you will get.)
DSLR = Digital Single Lens Reflex
ISO = a measurement standard for the sensitivity of the film or sensor to light. The higher the number, the more sensitive it is.
(ISO100 gives a better photo but is not very sensitive, ISO1600 may have a lot of “grain” but it is very sensitive)
Aperture = the opening where light can pass into the camera and onto the film or sensor.
Shutter Speed = the length of time that the camera shutter is open (on Canon it is the “Time Value”)
(the shutter is sort of a metal door that opens and closes, allowing or blocking light from entering the camera)
Exposure = the amount of light you have allowed to enter the camera
Overexposed = too much light (your photo is very bright/white or “blown out” )
Underexposed = not enough light (your photo is very dark)