What is a URI? It looks like the usage of this term has evolved. Around 1999, an article on URx said that "Uniform Resource Indicators (URI) are in development. URIs are defined as ASCII-based character string network protocols that represent communications streams... Identifiers are proxies for resources. A human being (resource) is labeled by a name (identifier), books are known by ISBN codes."

However the HTML 4.0 Recommendation and Terena's Guide to Network Resource Tools describe URI's as simply the protocol + host + path-within-host. Which leaves me wondering Is a URL any different from a URI? "URLs form a subset of the more general URI naming scheme."

The definitive clarification would be the w3C report URIs, URLs, and URNs: Clarifications and Recommendations 1.0 which "addresses how URI space is partitioned and the relationship between URIs, URLs, and URNs.... URL is a useful but informal concept: a URL is a type of URI that identifies a resource via a representation of its primary access mechanism (e.g., its network "location"), rather than by some other [identifying] attributes it may have. " By adding that "an http URI is a URL, " this puts our minds at rest -- we do not need to look for a distinction in http-space. "urn:isbn:n-nn-nnnnnn-n" is also a URI, but one based on an identifier (ISBN) rather than a location, and hence is not commonly referred to as a URL.

So we may as well just use the term URI universally, but there's nothing wrong with using URL in the context of location-oriented mechanisms like http. Some useful historical context is outlined at the http://www.w3.org/Addressing/.