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

messages: {
 required: "This field is required.",
 remote: "Please fix this field.",
 email: "Please enter a valid email address.",
 url: "Please enter a valid URL.",
 date: "Please enter a valid date.",
 dateISO: "Please enter a valid date (ISO).",
 dateDE: "Bitte geben Sie ein gültiges Datum ein.",
 number: "Please enter a valid number.",
 numberDE: "Bitte geben Sie eine Nummer ein.",
 digits: "Please enter only digits",
 creditcard: "Please enter a valid credit card number.",
 equalTo: "Please enter the same value again.",
 accept: "Please enter a value with a valid extension.",
 maxlength: $.validator.format("Please enter no more than {0} characters."),
 minlength: $.validator.format("Please enter at least {0} characters."),
 rangelength: $.validator.format("Please enter a value between {0} and {1} characters long."),
 range: $.validator.format("Please enter a value between {0} and {1}."),
 max: $.validator.format("Please enter a value less than or equal to {0}."),
 min: $.validator.format("Please enter a value greater than or equal to {0}.")
},

jQuery.extend(jQuery.validator.messages, {
 required: "必选字段",
 remote: "请修正该字段",
 email: "请输入正确格式的电子邮件",
 url: "请输入合法的网址",
 date: "请输入合法的日期",
 dateISO: "请输入合法的日期 (ISO).",
 number: "请输入合法的数字",
 digits: "只能输入整数",
 creditcard: "请输入合法的信用卡号",
 equalTo: "请再次输入相同的值",
 accept: "请输入拥有合法后缀名的字符串",
 maxlength: jQuery.validator.format("请输入一个 长度最多是 {0} 的字符串"),
 minlength: jQuery.validator.format("请输入一个 长度最少是 {0} 的字符串"),
 rangelength: jQuery.validator.format("请输入 一个长度介于 {0} 和 {1} 之间的字符串"),
 range: jQuery.validator.format("请输入一个介于 {0} 和 {1} 之间的值"),
 max: jQuery.validator.format("请输入一个最大为{0} 的值"),
 min: jQuery.validator.format("请输入一个最小为{0} 的值")
});

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

$().ready(function() {
 $("#signupForm").validate();
});


<form id="signupForm" method="get" action="">
 <p>
  <label for="firstname">Firstname</label>
  <input id="firstname" name="firstname" class="required" />
 </p>
 <p>
 <label for="email">E-Mail</label>
 <input id="email" name="email" class="required email" />
 </p>
 <p>
 <label for="password">Password</label>
 <input id="password" name="password" type="password" class="{required:true,minlength:5}" />
 </p>
 <p>
 <label for="confirm_password">确认密码</label>
 <input id="confirm_password" name="confirm_password" type="password" class="{required:true,minlength:5,equalTo:'#password'}" />
 </p>
 <p>
  <input class="submit" type="submit" value="Submit"/>
 </p>
</form>

class="{required:true,minlength:5,messages:{required:'请输入内容'}}"

class="{required:true,minlength:5,equalTo:'#password'}"

$().ready(function() {
 $("#signupForm").validate({
  rules: {
 firstname: "required",
 email: {
 required: true,
 email: true
 },
 password: {
 required: true,
 minlength: 5
 },
 confirm_password: {
 required: true,
 minlength: 5,
 equalTo: "#password"
 }
 },
  messages: {
 firstname: "请输入姓名",
 email: {
 required: "请输入Email地址",
 email: "请输入正确的email地址"
 },
 password: {
 required: "请输入密码",
 minlength: jQuery.format("密码不能小于{0}个字 符")
 },
 confirm_password: {
 required: "请输入确认密码",
 minlength: "确认密码不能小于5个字符",
 equalTo: "两次输入密码不一致不一致"
 }
 }
 });
});

<form id="signupForm" method="get" action="">
 <p>
  <label for="firstname">Firstname</label>
  <input id="firstname" name="firstname" />
 </p>
 <p>
 <label for="email">E-Mail</label>
 <input id="email" name="email" />
 </p>
 <p>
 <label for="password">Password</label>
 <input id="password" name="password" type="password" />
 </p>
 <p>
 <label for="confirm_password">确认密码</label>
 <input id="confirm_password" name="confirm_password" type="password" />
 </p>
 <p>
  <input class="submit" type="submit" value="Submit"/>
 </p>
</form>