Uniform Resource Location (URL) is the syntax and semantics for a compact string representation for a resource available via the Internet. For example, we use URL to locate web addresses and FTP site addresses. The generic syntax for URLs provides a framework for new schemes to be established using protocols other than those defined in this document.
URLs are used to `locate" resources, by providing an abstract identification of the resource location. Having located a resource, a system may perform a variety of operations on the resource, as might be characterized by such words as `access", `update", `replace", `find attributes". In general, only the `access" method needs to be specified for any URL scheme.
Protocol Structure
URLs are sequences of characters, i.e., letters, digits, and special characters. URLs are written as follows:
<scheme>:<scheme-specific-part>
A URL contains the name of the scheme being used (<scheme>) followed by a colon and then a string (the <scheme-specific-part>) whose interpretation depends on the scheme.
Scheme names consist of a sequence of characters. The lower case letters "a"--"z", digits, and the characters plus ("+"), period ("."), and hyphen ("-") are allowed. For resiliency, programs interpreting URLs should treat upper case letters as equivalent to lower case in scheme names (e.g., allow "HTTP" as well as "http").
Sponsor Source: URL is defined by IETF (http://www.ietf.org) RFC 1738
Reference:
http://www.javvin.com/protocol/rfc1738.pdf : Uniform Resource Locator (URL)