Well, as far as martial applications go, I have come to understand ki as a metaphor, or a loaded term that includes many things.
A good technique will come from a solid connection to the ground, proper skeletal alignment, a relaxed body, adequate concentration, proper breathing, and muscles which fire in the appropriate sequence so that the various body components all work together rather than in opposition. I have heard aikido people describe this as "total body coordination".
In other words, a good technique will have good ki. It's a handy word to throw out there when you want to talk about all these elements coming together without having to list them all. People tend to want to mystify ki because it just seems so much more attractive than doing all the hard work it takes to bring all the elements involved into play.
I won't speak to the more philosophical/religious aspects of Universal Ki or our collective being. Everyone has to make his or her own decisions about that.