references:
http://en.wikipedia.org/wiki/HTTP_cookie
http://wp.netscape.com/newsref/std/cookie_spec.html

basic sequence

//browser < server 
//response header
//Set-Cookie: A=a; B=b; Z=z
//document.cookie=="A=a; B=b; Z=z"
document.cookie = "name=value";
//document.cookie=="A=a; B=b; name=value; Z=z" //ordered by Ascend
document.cookie = "name=value2";
//document.cookie=="A=a; B=b; name=value2; Z=z" //3rd part cookie can be modified
document.cookie = "Z=z2";
//document.cookie=="A=a; B=b; name=value2; Z=z" //1st part cookie can't be modified
//maybe document.cookie=="A=a; B=b; name=value2;Z=z2;  Z=z" //1st part cookie can't be modified
//browser > server 
//request header
//Cookie: A=a; B=b; name=value2; Z=z

special names

Set-Cookie: NAME=VALUE; expires=DATE; path=PATH; domain=DOMAIN_NAME; secure

tips

• 多个ie进程使用各自的cookie,同一个ie进程的多个tab使用一个cookie。
• document.cookie是一个对象，使用其作为赋值操作符左值进行修改操作.
• document.cookie作为赋值操作符右值时,隐式调用其toString()方法。