I want this issue be settled for me once and for all.

How many types of pounds (mass unit) are there?

I just don't understand what ratio should I use to conver it to the metric system. I always thought that 1 pound = 0.45 kg, but I have doubts now, because a friend of mine insists that 1 pound = 0.466 kg

There are so many types of pounds, according to http://en.wikipedia.org/wiki/Pound_(mass) that I got confused completely.

When they say in a newspaper for example that something weights 90 pounds which pound are they referring to? What ratios should I use for conversion?

And do pounds in UK differ from pounds in USA?

I have the same question about miles. When they say a hundred miles - what exaclty it would be in kilometers?

And the last question - how do you people can remember all those differences without a headache?