The speed of your shutter - or how quickly it makes that snappy noise. Okay, okay, I know I sound silly when I say it like that, but that's kind of what it is. The higher the ratio 1/125 vs 1/250 the faster the shutter snaps down and captures your image. A fast moving image will need a faster shutter speed, but there's a caveat to that. The faster the shutter the less time you allow your camera to capture light - leading to a darker photo - you will have to adjust other settings to get the perfect setting. For long exposures reduce it to about as low as it can go and you can get awesome pictures like this :
The amount of light you allow into your lens. As the number goes higher - the less amount of light you allow into your lens. It's usually one of the easiest settings to change throughout shooting because it will constantly vary.
Iso Sensitivity is a little more tricky. The lower the number the crisper the image - but there are a lot of factors that will come into play when choosing your setting. Sometimes I just give up and click AUTO because nowadays the cameras are very smart and trusting them is better than fooling around with settings for 20 minutes and losing your shot.
For Further information and explanation go to digital-photography-school.com/tips