The word means: An event ,or a course of events, that will inevitably happen in the future.
To explain the word further than the basic lexically meaning, gets trickier.
however the personal meaning of the word may differ from person to person. I'd say that destiny is a powerful word with much feelings attached to it.
My conclusion is this:
If you believe in destiny, then there's not much you can do about it!
If you don't' ,well then you can change things by conscious decisions.
Destiny is not possible without fate (like Ovais said), to predict something with absolut certainty is to me impossible, some things are extremely likely perhaps, but nothing in the future (or present for that matter) is certain.