What's the difference bettween a 1:1.8 and1:3.5 camera lens.?

The 1.8 and 3.5 refer to the maximum aperture. 3.5 is what you would expect in most standard cheap lenses. 1.8 on the other hand is very, very good! The 1.8 is referred to as a 'fast lens' because at the low aperture of 1.8, a faster shutter speed can be used. Good for sports, wildlife and low light photography.

0:2:3

3.5 must be less expensive, it needs more light.

A 1.8 lens would be a faster lens, or allow more light in so you can use faster shutter speeds. It would also be a better quality and more expensive lens, all else being equal.

See this Wikipedia article for more information on lens speeds. http://en.wikipedia.org/wiki/Fast_lens

The first one is faster (wider aperture in relation to the focal length) the second on is "slower" for the same reason.
The first one is more expensive, but again how many times you use this aperture to justified the price ??

Those numbers refer to the widest aperture ratio (or f-stop) the lens can achieve. The aperture is the opening that allows light through. Remember, it's a ratio number, so the smaller the number, the wider the aperture. The wider the aperture, the more light can enter through the lens.

Your camera uses a combination of proper shutter speed and aperture to get correct exposure of light. Therefore, if you use a wider aperture, more light is coming in and your shutter speed will be faster to compensate for the extra light. This is important if you are shooting in lower light situations where a wide aperture will provide you with a fast enough shutter speed to keep your picture from blurring because of hand shake or movement of your subject.

There is also a correlation of "depth of field" and aperture. Depth of field is the range, near to far, of how much of the image will be acceptably sharp. Without explaining too much, a wider aperture provides less depth of field, meaning the foreground and background will be more out of focus. This is preferable when you want to make your subject stand out from it's background, such as in portraits of a person with a blurry background. A narrower aperture gives more depth of field, or allows more of the picture to be sharp. This is preferable in landscape photography, for instance, where the photographer wants as much of the image to be detailed and sharp.

Most zoom lenses are commonly in the f/3.5-6 range, with a zoom lens with a constant 2.8 aperture being considered a "fast" lens, and more expensive.

Primes lenses (or non-zoom) can be found commonly under f/2.8 and are much easier to produce (and cheaper) than zooms. Very fast prime lenses are usually considered around f/1.4.

It is the speed of the lens! the figures 1.8 and 3.5 are the maximum apertures or fstop available for that particular lens.
Assuming both lens have the same focal length say 55mm then.
lens A at 1.8 is faster and more expensive, it will take images in poorer light.
lens B at 3.5 is slower and less expensive it cannot take images in poorer light.
But remember at 1.8 you wont have much depth of field. (That which is focus front to back of the image.) Hope this helps!

1:1.8 for Telephoto lenses like 50mm f/1.8 and 1:3.5 for Zoom lenses like 28-300mm f/3.5 to 6.3.