what is a shutter?
a shutter is a metal 'curtain' in front of the camera sensor
when you press the button it opens and exposes the sensor to light
what does shutter speed control?
the slower the shutter speed, the longer it stays open for and therefore the more light it lets in. and vice versa.
this means that a photograph with a slow shutter speed of -say - 4 seconds is probably gonna be blurry unless you're shooting a very still subject
a photograph with a fast shutter speed of 1/2000th of a second can freeze a water droplet in focus mid-shot
downsides of very fast and very slow shutter-speeds:
very fast lets a lot less light in (potential underexposure), and very slow lets a lot more light in (potential overexposure) so you're going to want to compensate with a high ISO setting and/or a low aperature setting
the difference between blur and camera shake:
blur = your subject moved during shooting
camera shake = your hands moved during shooting