Is there anything like that links should be given inside(outside) the <form></form> tags. Can form tag be left without any action?

That is not a JSP question, technically, and this is the wrong forum for a JSP question, anyway, there is a JSP forum, that is an HTML question. The real question is, why would you want a form without an action? What purpose would it serve? That is just some food for thought. My advice would be to find an HTML forum.

O.k. i surfed and found that a login.jsp(form based Authentication) page needs some code to be entered in web.xml. I submit my code here.

<html>
<head>
<title>Enter your name and password</title>
</head>
<body bgcolor="#999966">
<p>&nbsp;</p>
<form method="POST" action="Order.jsp">
<p><font color="#800000" size="5">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
Enter your company  name:</font><input type="text" name="coname" size="20"></p>
<p><font color="#800000" size="5">
Enter your password:</font><input type="password" name="password" size="20"></p>
<p><input type="submit" value="Submit" name="B1"></p>
<% String cname= request.getParameter("coname");
   String cpwd= request.getParameter("password");
   Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
   Connection c=DriverManager.getConnection("Jdbc:Odbc:emp");
   Statement s= c.createStatement();
   ResultSet r=s.executeQuery("select * from complog where Comp_name="+cname+" ");
   while(r.next())
      {
        String cpwd1 =r.getString(2);
        if(cpwd1.equals(cpwd))
          {
            response.sendRedirect("Order.jsp");
          }
%>
</form>
</body>
</html>

I felt I have use redirect there which is wrong.

Don't use scriptlets in JSP and use a Connection Pool. And you can't use a "sendRedirect" in the middle of a page anyway. A sendRedirect needs to be the entire page.

Also, form-based authentication is normally handled by the web container itself, you, normally, need only provide an html form and the connection information to the user realm.

See this and this.