之前只模糊的记得URL是URI的子集，今天看了个链接总算清楚了。链接http://www.bernzilla.com/item.php?id=100
"URI stands for Universal Resource Identifier and URL stands for Universal Resource Locator. Often times people use the terms interchangably, which is not entirely correct. A URL is a subset of the URI popular protocols. These are protocols (http://, ftp://, mailto:). Therefore all URLs are URIs. The term URL is deprecated and the more correct term URI is used in technical documentation. All URIs are means to access a resource on the Internet and are a a technical short hand used to link to the resource. URIs always designate a method to access the resource and designate the specific resource to be accessed."
原来URL是URI常用协议的一个子集。URI指定了访问资源的方法和被访问的资源；而URL从字面上理解来讲，只是资源的位置，所以它是在URI的一些协议下的子集。