URIs identify and URLs locate; however, locators are also identifiers, so every URL is also a URI, but there are URIs which are not URLs.
Examples
This is my name, which is an identifier.
It is like a URI, but cannot be a URL, as it tells you nothing about my location or how to contact me.
In this case it also happens to identify at least 5 other people in the USA alone.
- 4914 West Bay Street, Nassau, Bahamas
This is a locator, which is an identifier for that physical location.
It is like both a URL and URI (since all URLs are URIs), and also identifies me indirectly as "resident of..".
In this case it uniquely identifies me, but that would change if I get a roommate.
I say "like" because these examples do not follow the required syntax.
Popular confusion
From Wikipedia:
In computing, a Uniform Resource Locator (URL) is a subset of the Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it. In popular usage and in many technical documents and verbal discussions it is often incorrectly used as a synonym for URI, ... [emphasis mine]
Because of this common confusion, many products and documentation incorrectly use one term instead of the other, assign their own distinction, or use them synonymously.
URNs
My name, Roger Pate, could be like a URN (Uniform Resource Name), except those are much more regulated and intended to be unique across both space and time.
Because I currently share this name with other people, it's not globally unique and would not be appropriate as a URN. However, even if no other family used this name, I'm named after my paternal grandfather, so it still wouldn't be unique across time. And even if that wasn't the case, the possibility of naming my descendants after me make this unsuitable as a URN.
URNs are different from URLs in this rigid uniqueness constraint, even though they both share the syntax of URIs.
Best Answer
The URI RFC doesn't mandate a format for the query string. Although it is recognized that the query string will often carry name-value pairs, it is not required to (e.g. it will often contain another URI).
HTML establishes that a form submitted via HTTP GET should encode the form values as name-value pairs in the form "?key1=value1&key2=value2..." (properly encoded). Parsing of the query string is up to the server-side code (e.g. Java servlet engine).
You don't identify what server-side framework you use, if any, but it is possible that your server-side framework may assume the query string will always be in name-value pairs and it may choke on a query string that is not in that format (e.g.
?bar
). If its your own custom code parsing the query string, you simply have to ensure you handle that query string format. If its a framework, you'll need to consult your documentation or simply test it to see how it is handled.