Syntax:	root path;
Default:	
root html;
Context:	http, server, location, if in location
Sets the root directory for requests. For example, with the following configuration

访问 http://localhost:80/i/top.gif,nginx匹配的文件路径是/data/w3/i/top.gif

location /i/ {
    root /data/w3;
}

The /data/w3/i/top.gif file will be sent in response to the “/i/top.gif” request.

The path value can contain variables, except $document_root and $realpath_root.

A path to the file is constructed by merely adding a URI to the value of the root directive. If a URI has to be modified, the alias directive should be used.

---

Syntax:	alias path;
Default:	—
Context:	location
Defines a replacement for the specified location. For example, with the following configuration

访问http://localhost:80/i/top.gif,文件路径为/data/w3/images/top.gif。
这里有个细节，nginx在解析root时会自动加上/,因此root可以省略斜杠，但alias路径结尾需要加上斜杠才能正确解析。

location /i/ {
    alias /data/w3/images/;
}

on request of “/i/top.gif”, the file /data/w3/images/top.gif will be sent.

The path value can contain variables, except $document_root and $realpath_root.

If alias is used inside a location defined with a regular expression then such regular expression should contain captures and alias should refer to these captures (0.7.40), for example:

location ~ ^/users/(.+\.(?:gif|jpe?g|png))$ {
    alias /data/w3/images/$1;
}

When location matches the last part of the directive’s value:

location /images/ {
    alias /data/w3/images/;
}

it is better to use the root directive instead:

location /images/ {
    root /data/w3;
}

nginx的官方文档:
http://nginx.org/en/docs/http/ngx_http_core_module.html#alias
http://nginx.org/en/docs/http/ngx_http_core_module.html#root