<script type="text/javascript" src="jquery.js"></script>
<script type="text/javascript" src="jquery.validate.js"></script>

.error{
  color:red;
  margin-left:8px;
}

$(document).ready(function() {
  // validate signup form on keyup and submit
  var validator = $("#signupform").validate({
    rules: {
      firstname: "required",
      username: {
        required: true,
        minlength: 2
      },
      password: {
        required: true,
        minlength: 5
      },
      password_confirm: {
        required: true,
        minlength: 5,
        equalTo: "#password"
      },
      email: {
        required: true,
        email: true,
      },
      dateformat: "required",
      terms: "required"
    },
    messages: {
      firstname: "姓名不能为空",
      username: {
        required: "用户名不能为空",
        minlength: jQuery.format("用户名只少由 {0} 字符组成")
      },
      password: {
        required: "密码不能为空",
        minlength: jQuery.format("密码只少由 {0} 字符组成")
      },
      password_confirm: {
        required: "确认密码不能为空",
        minlength: jQuery.format("确认密码只少由 {0} 字符组成"),
        equalTo: "秘密与确认密码不一致"
      },
      email: {
        required: "邮箱不能为空",
        email: "邮箱格式不正确"
      },
      dateformat: "请选择性别",
      terms: " "
    },
    // the errorPlacement has to take the table layout into account
    errorPlacement: function(error, element) {
      if ( element.is(":radio") )
        error.appendTo( element.parent().next().next());
      else if ( element.is(":checkbox") )
        error.appendTo ( element.next());
      else
        error.appendTo( element.parent().next());
    },
    // specifying a submitHandler prevents the default submit, good for the demo
    submitHandler: function() {
      alert("submitted!");
    },
    // set this class to error-labels to indicate valid fields
    success: function(label) {
      // set   as text for IE
      label.html(" ").addClass("checked");
    },
    highlight: function(element, errorClass) {
      $(element).parent().next().find("." + errorClass).removeClass("checked");
    }
  });
});

$.validator.addMethod("checkUserName", function(value) {

    //value为验证的值，对应于元素id

  //方法代码

}, '用户名格式不正确');

<form id="signupform" autocomplete="off" method="get" action="">
   <table>
   <tr>
    <td class="label"><label id="lfirstname" for="firstname">姓名</label></td>
    <td class="field"><input id="firstname" name="firstname" type="text" value="" maxlength="100" /></td>
    <td class="status"></td>
   </tr>
   <tr>
    <td class="label"><label id="lusername" for="username">用户名</label></td>
    <td class="field"><input id="username" name="username" type="text" value="" maxlength="50" /></td>
    <td class="status"></td>
   </tr>
   <tr>
    <td class="label"><label id="lpassword" for="password">密码</label></td>
    <td class="field"><input id="password" name="password" type="password" maxlength="50" value="" /></td>
    <td class="status"></td>
   </tr>
   <tr>
    <td class="label"><label id="lpassword_confirm" for="password_confirm">确认密码</label></td>
    <td class="field"><input id="password_confirm" name="password_confirm" type="password" maxlength="50" value="" /></td>
    <td class="status"></td>
   </tr>
   <tr>
    <td class="label"><label id="lemail" for="email">邮箱</label></td>
    <td class="field"><input id="email" name="email" type="text" value="" maxlength="150" /></td>
    <td class="status"></td>
   </tr>
         <tr>
    <td class="label"><label>性别</label></td>
    <td class="field" colspan="2" style="vertical-align: top; padding-top: 2px;">
    <table>
    <tbody>

    <tr>
      <td style="padding-right: 5px;">
        <input id="sex_men" name="dateformat" type="radio" value="0" />
        <label id="lbl_sex_men" for="dateformat_eu">男</label>
      </td>
      <td style="padding-left: 5px;">
        <input id="sex_women" name="dateformat" type="radio" value="1" />
        <label id="lbl_sex_women" for="dateformat_am">女</label>
      </td>
      <td>
      </td>
    </tr>
    </tbody>
    </table>
    </td>
   </tr>

   <tr>
    <td class="label"> </td>
    <td class="field" colspan="2">
      <div id="termswrap">
        <input id="terms" type="checkbox" name="terms" />
        <label id="lterms" for="terms">以阅读并同意网站条款.</label>
      </div> <!-- /termswrap -->
    </td>
   </tr>
   <tr>
    <td class="label"></td>
    <td class="field" colspan="2">
    <input id="signupsubmit" name="signup" type="submit" value="注册" />
    </td>
   </tr>

   </table>
</form>