Disclaimer: these are my own thoughts about grief - sometimes they coincide with what modern science says, sometimes not. What you find here is the result of lots of reading (I read a LOT about it because I wanted to understand badly what was going on in me and to get rid of the pain as soon as possible), thinking, struggles and debates with myself. The bad news is that there is no highway to inner peace after losing someone who was part of your life - but you already know this I guess. However, there is good news, too: as you understand more and more of how we human beings are put together you will go through a transformation that finally gives you the peace you search for. At least, this is what happened to me, your fellow human who is not any more special than you are.
Disclaimer 2: English is a second language for me. I apologize for my grammar, I am sure there will be places where I do not use the proper words but I do not have an editor or proof-reader - I just cannot afford it. So please make an intelligent guess of what I am trying to say instead of what I actually write, and just jump over (or help me to correct it ;) )
The term "grief" gives 169 million hits on Google in 0.3 second. If you go to Amazon.com, it gives you 50,000+ hits, so I will not pretend to have a unique never-yet-said definition for it. Simply put: it is the normal reaction to a loss.
Mourning is what we do outside while we are in grief.
Complicated or prolonged grief refers to a condition when someone is not able to adapt to recover from a loss within a time frame of 6 months (!). It is considered to be a disorder.