峡山水库蚂蚱:帮忙注释一下代码

这是用户注册时如果注册的用户名已存在的情况
帮忙注释一下,有的看不太懂.

dim admin,username,password
username=trim(request("username")) '从login.asp取出文本框的值
password=trim(request("password"))
set rs=server.createObject("adodb.recordset") '建立记录集
sql="select * from user where username='"&trim(request.Form
("username"))&"'"
rs.open sql,conn,1,3

if not rs.eof then '如果不到最后一条记录,说明已经存在该用户
response.write ("<script language=javascript>")
response.write ("alert('用户名已经存在,请重新输入名字!');")
response.write ("history.go(-1);")
response.write ("</script>")
response.End
else
rs.addnew
rs("username")=username
rs("pass")=password
rs.update
rs.close
session("admin")=0
response.write ("<script language=javascript>")
response.write ("alert('注册成功');")
response.write ("history.go(-2);")
response.write ("</script>")
response.end
end if