The answer is a lot and yes it does matter.
People often forget the extension, the “.com”, “.co.uk”, “.uk” part (correctly referred to as the “top-level domain”). So if you can get the main ones when registering a domain it is worth doing, mainly so that your potential customer doesn’t end up at one of your competitors websites.
If, for example, you are a UK plumbing company thinking of registering “MyNamePlumbing” we would recommend you register:
mynameplumbing.com
mynameplumbing.co.uk
mynameplumbing.uk
Domain Extensions explained:
.com – originally intended for use by commercial/for-profit business entities, although is now used by many others, simply because it is one of the oldest and most well-known.
.co.uk – intended for use by UK businesses.
.uk – intended to help shorten domain names of UK businesses.
.org – originally intended for use by non-profit organisations and still primarily used by them.
There are a lot of other top level domains and if you’re interested in more information please visit http://en.wikipedia.org/wiki/List_of_Internet_top-level_domains