Ki, I think the closest translation of the word ki in english is energy. I believe that their are two main kinds of ki, personal ki and universal ki, what I like to call "The Flow".
Personal Ki is what I think every living and some nonliving things have inside them. Without it these things would cease to live. I believe it is a lot like electricity, just it is in your body and you are the generator of it. Scientists call it bio-electricity, it is what runs through your nerves and gives your brain information, and tells your muscles when to contract and relax, which enable you to move. I also think that adrenaline is a form of personal ki. However this is more like your backup or emergency only ki. You can learn to tap into it at will, with the proper training. I also think that ki is at least part of your soul in a semi-physicall form, I don't really know how to explain this yet.
Universal Ki (The Flow) is the energy of the universe, I think this type of ki is more like water than electricity. This kind of ki, unlike personal ki, I do not think you can control. You can direct it, but I think that it goes more where it wants, much like water. I think that the flow is everywhere, stronger in some areas and weaker in others, and it also flows at different speeds in different places. I think the flow is much like water and people and other objects are like pebbles in the water. They all give off ripples and if you are in harmony with the flow you can feel these ripples, and alter your own ripple. I think that to become harmonious with the flow is to lose yourself in it, or perhaps drown in it would be a better way of putting it. I don't know how to do this, and I don't think its something you can consciously do, it just happens.