Need a quick crash course on filters

I've just got a 450D kit with a HOYA ND4 Neutral Density filter. I'm thinking of getting a Circular Polarizer and perhaps just a plain UV filter. Is the aperture of the kit lens big enough to take advantage of the Neutral Density filter? Is it ok to use the ND filter for normal use?

From what I understand, a Circular Polarizer reduces the amount of light going into the camera so the background will not be washed out and reduce glare/reflection. Will it be ok to use it during daytime under cloudy/light smog condition?

Is a UV filter is good enough for night shots?

Thanks in advance

In most cases, you do NOT want to use a ND filter for normal photography, especialy if you are in a area where there isnt much light, like inside a house. The entire purpose of a ND filter is to reduce the ammount of light hitting your sensor, which in most cases you DONT want.

The primany purpose of a UV filter is to protect your lens. Cheaper UV filters can often cause more flare, and loss of sharpness in your shots. Use of a UV filter will almost never have a benificial effect on your shots. That being said i use one on all my lenses to protect the front element.

Protective Filter FAQ covers the clear filters left on all the time. The points made in there on filter quality will apply to any filter you may be interested in.

You can use the ND filter any time you need a longer exposure or faster aperture than you can get with just the camera's settings (ISO, aperture and shutter speed). A polarizer will also offer you about 2 stops of exposure shift, plus it'll also help improve colour saturation by reducing glare off non-metallic surfaces. It won't keep the background from being washed out if the wash out is happening because the background is overexposed. But if you're shooting at right angles to the sun, a polarizer can cut out a lot of glare in the sky.

http://www.dpfwiw.com/​filters.htm#why

http://www.2filter.com​/faq/faq.html

About the only time I will use a ND filter is when I want to slow my shutter speed down to blur the effect of running water.

I use a polarizing filter a lot. It enhances images in many ways if the angle of the light source is correct:

Reduces eliminates/reflections on non-metallic surfaces

This reduction/elimination can among other benefits:
Allow you to see below surface of water
Allow you to see through glass such as windows
Enhance the color saturation of foliage
Somewhat penetrate haze/fog by reducing reflections from particulates in air

Darkens the sky accentuating clouds and often bringing the image with in the dynamic exposur range of your sensor

It can aid in reducing exposure allowing slower shutter speeds to blur water