When the internet started it was used by a just few different organizations - mainly universities and government agencies, but also some companies.
To show what type of web site it was the last few letters denoted this, so COM was a company, GOV was goverment agencies, EDU was educational, and ORG was an organization.
As the internet grew lots more of these were added (called Top Level Domains), including ones for individual countries, like say UK for United Kingdom and CA for Canada and so on.
So generally COM is a worldwide company (Microsoft say) -although not always.
CO.UK is a UK Company, and ORG.UK is a UK organization.
NET is a company involved in the Internet Network (Virgin.NET for example)
While these can be taken as a guide I think anyone can create a web site with any address, so I could create one called www.bigcompany.com even though I am running the company from my bedroom.
You can see a list of all the top level domains here
http://en.wikipedia.org/wiki/List_of_Internet_ top-level_domains