Let's say you have written a script and you want to pitch it to a studio... should it be spec or shooting script?
And also... why do they both exist? Whats the point? In what type should I write?
OK... so, what's obvious from your questions is that you're fairly new to the industry. So, I'm going to read between the lines and answer the questions I think you're asking.
So, as has already been stated "Spec" means Speculative... all that means is you're not being paid to write it, but you are writing it to pitch ... as you've mentioned pitching, you're already writing a "spec"
Spec scripts are written to be read... shooting scripts are used for production.
When you ask what font to use for your script, that tells me a couple of things -- one that you're intending to write it in a standard piece of word processing software and secondly that you've never read any scripts.
Based on those assumptions, I'm going to give you three pieces of advice:
1)) Read some classic film scripts (you can find them online) -- I suggest you do this because film scripts are written in a very specific way and if you're asking basic font questions, then there are hundreds of other things you are bound to get wrong.
2) Either buy or download some screen writing software -- Celtx is a free one for the mac and there are loads of other options for the PC (this will default to the right font for a spec and will help with other basic format issues)
3) If you want to pitch to a studio, don't send your script to them until you're sure it'll stand a chance of being read.
I strongly suggest you read this
SCRIPT CHECKLIST before you even consider posting out your masterpiece.