HTTP = Hyper Text Transfer Protocol 超文本传输协议
URI = Universal Resource Identifier 统一资源标识符，可以唯一标识一个资源。
URL = Universal Resource Locator 统一资源定gaishenm位符，可以提供找到该资源的路径。例如：如何确定某一个人？
uri : 张三身份证号 xxxxxxxxxxxxxxxx
url : 银河系/太阳系/地球/亚洲/中国/北京/三里屯/张三
总之：locators are also identifiers, so every URL is also a URI, but there are URIs which are not URLs.