When you take a picture with any camera, there are three key ingredients that work together to produce the picture that you see on film or your digital media. They are aperture, shutter speed, and ISO. A different combination of these ingredients can produce vastly different results without even a change in camera position.
Well start briefly with ISO. ISO actually stands for International Standards Organization which on its own does not really tell you much about what it does. Basically, it measures the films sensitivity to light. A high ISO number means that the film will be more sensitive to light. While this can enhance the ease of taking the photo, especially in low light situations, it also results in more grain. A low ISO number means that the film is less sensitive to light. This does help avoid that grain.
The aperture determines the amount of light let through the lens and onto the film (or digitial media if you are using a digital camera). It is an opening in the lens itself made up of six overlapping blades. Setting the aperture size is done manually or automatically, depending on the capabilities of your camera. Depth of field is one of the primary things you can change by changing the aperture setting. A wide open aperture results in the background being blurred. A smaller aperture will result in sharper detail in the background.
Lastly, shutter speed dictates how long the shutter remains open. The shutter speed is primarily used to help control the effects of motion. A fast shutter speed will result in the action being frozen. A slow shutter speed will result in the moving subject being blurred. This is often done to help convey a sense of motion. You can see the color blur the direction the object is moving.