Filters are a useful aid to photographers. They come in many forms and essentially sit infront of your lens to either colour, darken or enhance your photograph.
The ND10 is a bit of a speciality filter and really takes a while to master - which I'm nowhere near yet. ND stands for Neutral Density and it blocks light without changing the colour that comes through the lens onto the sensor. The ND10 blocks ten stops of light which there for only lets in 0.1% of the light.
What's the point of this? Well it means that in broad day light you can keep the shutter open for ages. Normally this would make your shot completely white but with the filter in front you can easily do a 30 second shot without this happening. This has two significant effects.
First, clouds will move in that space of time creating some interesting skies. Secondly, water smooths out to give surreal textures. If you compare this shot with the first one you will see how different they are. This one was taken WITHOUT the ND10 filter and its more of a snapshot in time where as the upper picture really demonstrates the crazy effects you can pull off with this filter.
It's a nifty bit of kit but there are a few issues you should be aware of. It is so black that almost no light goes through and the camera really struggles to focus in autofocus mode. Sometimes, if it's bright enough and your camera has it, you can use Live View mode to focus but realistically you have to compose your shot first then put the filter on and take the shot. REMEMBER to turn off you auto focus after setting up the shot or else the camera will start trying to focus which you press the shutter and most likely it won't take the shot.
Another problem is that due to the super-dark nature of this filter it can't really be used long after sunset without having to leave the shutter open for minutes or longer. That presents another issue as you may then require a remote to lock the shutter open for more than 30 seconds - the tech and gadgetry of photography never ends I'm afraid. There are weaker filters that you can use, but alas I don't have any of those yet, but as soon as I do I'll post the results.
PS - A circular polariser can block 1 - 2 stops of light and can come in useful when the light is getting too low for the ND10 if you don't have any other filters.