How to choose neutral density filter?

I have 18-55mm and 70-300mm nikon lens for my nikon d50 camera. How do i know which neutral density filter will fit these lens?
I see many lens of various size eg:58mm ND8 OR ND2 ND4 72mm
i'm not sure what these number with and… Is it and level?

Added (1). My lens is 70-300 mm and another regular 18-55mm… How do you say 62 or 67 mm fits for 70-300mm lens? Why not 70mm or even higher within 70 - 300 mm range would not fit?
i want to know exactly what number one should choose for lens…

The 18-55 takes 52mm filter, The 70-300 either 62 or 67mm, depends on which one you have.

The digit behind and is usually how many stops difference it's going to make.

Here is an explaination of and filters:

http://www.althephoto.com/concepts/filters.php

You buy a filter according to the filter size your lens requires (in mm), then how much light you want to reduce. They are listed in EV (exposure value).

So for example, a 58mm ND8 would fit a 58mm lens thread, and have an exposure value of 1/8. Which means it lets in 1/8th as much light (-3EV).

Each EV lets in 50% less light. So a ND2 will let in one-half, or 50% as much light (-1EV), a ND4 one-fourth, or 25% as much (-2EV), a ND8, one-eighth, or 12.5% as much (-3EV), and so on.

While the numbering scheme seems a bit odd, if you realize that light behaves due to the inverse square law, it makes sense. The inverse square law is based on the power of 2.