Calling all English speaking people

What are countable nouns? Can someone please explain that to me? *angel face* I'm studying for the Cambridge Proficiency Exam right now. *nervously biting my fingers*

Help me please!

I did a practice essay for the Proficiency exam which I am taking tomorrow (keep your fingers crossed! :D And wish me good luck tomorrow if you want, believe me when I say that I'll need it). Anyway, I took the subject from my Proficiency workbook which was Space exploration: an investment in the future? on 300-350 words and well... I need your input. On the language but most importantly on the structure and if it's a good essay in itself, regardless of its subject.

( Space exploration: an investment in the future? - Practice essay - please read and help me )