What Does Your tld Tell Google?
Because I live in the UK, I usually buy only .co.uk or .com domain names.
It struck me as odd that some of my .com domains were ranking well (often number 1) on Google.com searches, but were not getting much exposure on Google.co.uk when searching for UK results.
Having researched this more lately, I thought I’d post my findings, since I’ve seen the question of ranking come up often in some of the forums I’ve visited. If you already know this stuff, then apologies for wasting your time.
Google takes a couple of things into consideration when looking at ranking for particular countries and on their own site, they explain that the cctld is indeed an important factor.
A .co.uk name is automatically geo-targeted for, or associated with a UK audience.
However, if you’re using a .com domain name, then it’s important to tell Google that you’re targeting a specific country audience by creating a Google account and logging into Google webmaster tools.

Google's instructions on how to set up geo-targeting of a .com or .net tld to a specific country audience
This Google video also explains the process further:
The tld isn’t the only consideration, as Google also looks at the IP address of where a site is hosted. If like me, you host your sites on US based servers, this spells double-trouble, so geo-targeting becomes all the more important.
Personally, I use .co.uk domain names for my most important sites from where I sell services, as I usually want only UK based clients or customers.
If like me, you’ve had difficulty in getting decent search engine exposure for your .com names in a non-US country, you might want to look into this.
Setting up geo-targeting is very simple and takes only a few moments.
Thank’s for the Valuable information David, I was not aware of this feature before reading your post, excellent stuff!