There are three things that affect exposure in photos (excluding ND filters): ISO, Aperture, and Shutter Speed.
ISO is the sensitivity of the sensor/film. Low ISO requires more light (longer shutter speed or larger aperture), but yields better images with less noise and better tonality. Higher ISO requires less light, but sacrifices image quality.
Aperture is the opening in your lens that allows light to hit the sensor. This is used to control depth of field and sharpness. To isolate your subject from the background, shoot with a larger aperture (smaller f-number). For optimum sharpness in the whole frame, you would usually stop down to around f8 or f11 and focus near infinity ( most lens' sharpness is limited by refraction).
Shutter speed controls motion blur. To freeze action or for sharper handheld shots, use a fast shutter speed. Slow shutter speeds are used if you want motion blur in your image, or to allow you to use a lower ISO/smaller aperture when on a tripod in lower light.
So basically the relationship between the three factors can be summed up in whats called the Exposure Triangle:
