<n*>
  <ul class="n*bar">
    <li><a href="#">首页</a></li>
    <li class="dropdown">
      <a href="#">产品</a>
      <ul class="dropdown-menu">
        <li><a href="#">电子产品</a></li>
        <li><a href="#">家居用品</a></li>
        <li><a href="#">服装鞋帽</a></li>
      </ul>
    </li>
    <li><a href="#">服务</a></li>
    <li><a href="#">关于我们</a></li>
  </ul>
</n*>

.n*bar {
  list-style: none;
  margin: 0;
  padding: 0;
  background-color: #333;
  overflow: hidden; /* 清除浮动影响 */
}
<p>.n*bar li {
float: left; /<em> 水平排列菜单项 </em>/
}</p><p>.n*bar a {
display: block;
color: white;
text-decoration: none;
padding: 14px 20px;
}</p><p>.n*bar a:hover {
background-color: #555;
}</p>

.dropdown-menu {
  list-style: none;
  margin: 0;
  padding: 0;
  position: absolute;
  background-color: #444;
  min-width: 160px;
  display: none; /* 默认隐藏 */
}
<p>.dropdown-menu li {
float: none; /<em> 子菜单垂直排列 </em>/
}</p><p>.dropdown-menu a {
padding: 12px 16px;
}</p><p>.dropdown:hover .dropdown-menu {
display: block; /<em> 悬停时显示下拉菜单 </em>/
}</p>

.dropdown {
  position: relative; /* 为子菜单提供定位上下文 */
}
<p>.dropdown-menu {
top: 100%;
left: 0;
z-index: 100;
box-shadow: 0px 8px 16px rgba(0,0,0,0.2);
}</p>