What is a URI?
A URI is an address or 'identifier' for a specific file or resource on the internet. A URI provides the instructions to allow computer programs to find the file you have requested. Since URIs are "uniform" in format, you can use the same URI in different programs (like Opera or Firefox) to access the same resource.
For all practical purposes for most small business owners, a URI is the same as a URL or a web address. The URI for your web site will be your domain name.
When you get into the networking semantics, there are differences between URLs and URNs, both of which are types of URIs.