What is time?

What is time?  Is it just the way we measure our lives? 
What is past, present or future?  Is it the order that events must take place? 
Do events take place one after the other, a neverending linear sequence of history? Or do they take place all at once?  The past cannot be changed, that much we know, but is the future? Do our actions have any impact on the future are is that also set in stone?  Do we have any freewill at all, or is everything we do prewritten. Ordained by some greater plan that we are helpless to stop? 
If this really is all controlled, then why would the Greater Being allow us to have these thoughts at all?  If we do write our own stroies, our own futures, then we could never really know that we are doing so. No one can see what would have happened, we can only see the present. 
We live in one moment at a time, yet thousands are happening at the same instant. We don't know what is happening in places we aren't, we may never find out about these events at all yet the impact they can have on our lives is immense. Or maybe they won't affect us at all. 
Time is strange that way, I suppose, we can never really understand it.  Yet we must live it.