Originally, the three letter extensions after the dot (or Top Level Domain/TLD) were meant to denote whether the domain name was being used for business (.com) charity/non-profit (.org) or for a Technology based company (.net). However, with the explosion of the Internet (and specifically, the world wide web) as a new business medium, the lines were blurred, and companies and individuals alike started cross-registering domains (ie. Me.com, me.net, me.org) just to protect their interests. Now, .com, .net, and .org names (the generic Top Level Domains) can be used for any purpose.
Dot com(.com) refers to a commercial website. dot co (.co) would be sites at specific countries. Say co.in (India) co.uk(United Kingdom) co.nz(New zealand). For eg. www. google.com opens as www.google.co.in in India.
If you are getting the dot com, you are getting the url that people can and will remember the best. The only remote exception may be charities and organisations that may use dot org, though i still think they need the dot com and use 301 to the dot org.