Oh Dear...wherec to start!!! j/k. I was like that when I first strated looking at diamonds.....and there is a lot to learn before you even half know what the real deal is!!!
I strongly suggest you go to a diamond site like Pricescope and read up on all the info you can manage...its infinate!
I will give you a short summary to start you off. First, the specs are only as good as the lab. who issued the report. The labs work for the seller and not the buyer so keep that in mind. Some labs will be very soft in their report so a J may be a H in the report etc.
Keep in mind that cut is king. The diamonds cut and polish is the only thing that lets it sparkle...otherwise it is just a rock! Now, the angles and relative lengths of the facets is what gives the diamond its scintilation, fire, brightness sparkle etc. The facets act as windows and mirrors....and to be honest it gets very technical, but 10% of diamonds are cut for max. performance, and the rest are cut to different degrees of great to average etc.
A great cut and polish will make a diamond look whiter and actually hide imperfections better (in the blinding flashes)!
Budget is usually the biggest hurdle to all of us women getting the 5 carat D IF diamond that we all deserve.
Fancy cuts like the pear have to be seen on an individual basis to determine if its beautiful, the specs wont tell the whole story. Usually this applies to all diamonds actually as sometimes even with perfect specs the diamond may not face up as well as another with less perfect specs.
Some diamonds are cut more symetrically (h& a) and some more randomly, this boils down to preferance as each gives a different scintilation effect.
The clarity refers to what the eye can see from the top of the diamond. If you are lucky you can get a low grade clarity that is still eye clean for a better price, or more carat.
Flourecence will pull the price down, make a yellow diamond look whiter, make a white diamond look `blue`.....but watch out because sometimes it causes cloudiness, mostly not!
My honest opinion is that most diamonds once set will look great even if the cut is not perfect. And in a jewellery shop ALL diamonds will look amazing, its the lighting. Diamonds need to be clean and have proper lighting to perform at their best!