Basically what it does is, depending on the strength you use, gives you several stops to work with. In my case, I have the darkest ND filter you can get, which gives me 3 stops. What this means is, I can shoot for long exposure's in daylight.
1. I scout the shot.
2. I set the camera lens on Auto focus (while setting the camera itself on manual).
3. I autofocus the scene, then switch the lens back to manual focus
4. I set my exposure time to about 10 seconds.
5. I screw the ND filter on the lens.
6. I set the self-timer (cause I often forget my remote)
7. I press the release button
8. I wait 
So, those images above were taken with an ISO 100, f/7.1 1/10 using my 12-24mm lens.
In doing this, water becomes silky and if the clouds are moving, they kinda blur. You can also use a circular polarizer to take reflections off the water if you want too but I like the reflections.
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