How to create popup boxes next to the links when mouse over them?

Here is the simpliest solution.

HTML:

<a href="http://foo.com" data-tooltip="#foo">foo</a>
<a href="http://bar.com" data-tooltip="#bar">bar</a>

<div id="foo">foo means foo</div>
<div id="bar">bar means bar</div>

CSS:

div {
    position: absolute;
    display: none;
    ...
}​

JavaScript:

$("a").hover(function(e) {
    $($(this).data("tooltip")).css({
        left: e.pageX + 1,
        top: e.pageY + 1
    }).stop().show(100);
}, function() {
    $($(this).data("tooltip")).hide();
});

$("a").hover(function(e) {
  $($(this).data("tooltip")).css({
    left: e.pageX + 1,
    top: e.pageY + 1
  }).stop().show(100);
}, function() {
  $($(this).data("tooltip")).hide();
});

div {
  position: absolute;
  display: none;
  border: 1px solid green;
  padding:5px 15px;
  border-radius: 15px;
  background-color: lavender;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.0.0/jquery.min.js"></script>

<a href="http://foo.com" data-tooltip="#foo">foo</a>
<a href="http://bar.com" data-tooltip="#bar">bar</a>

<div id="foo">foo means foo</div>
<div id="bar">bar means bar</div>

​DEMO: http://jsfiddle.net/8UkHn/

Have you considered using a "title" attribute in this case?

<a href="http://foo.com" title="foo means foo"> foo </a>

See if this fits your need.

What it does is, when you move mouse over the link "foo", a small box containing "foo means foo" will appear next to the mouse pointer.