All settings affect how much light enters the camera. But each one has a specific effect on certain areas.
Aperture
Aperture controls depth of field (aka background blur).
The higher the f-stop, the less depth of field you get but the darker the image. The lower the f-stop, the brighter the image and the shallower the depth of field. This can mean the edges of your subject get blurred and cutting them out in Photoshop becomes harder.
Aim for f.5 when taking photos in the studio for a nice edge to cut out.
Shutter Speed
Shutter speed controls motion blur.
ISO
ISO controls image noise.
Adjusting one for a specific effect means you have to adjust the other two for proper exposure.