The most simple way is to use the 'title' attribute like this...

HTML
<span title="Example of the title attribute in use">like this</span>

CSS
a.tooltip {
position:relative;
cursor:help;
}
a.tooltip span {
display: none;
position:absolute;
top:1.5em;
left:0;
width:15em;
padding:0 2px;
}
a.tooltip:hover {
display:inline;
}
a.tooltip:hover span {
display:block;
border:1px solid gray;
background-color:white;
}

HTML

<a class="tooltip" href="#n">Karl Marx<span>-info goes here-</span></a>

Without this part... a.tooltip:hover {
display:inline;
}

..it won't work in IE.

The "#n" in the link is to prevent the page from jumping to the top if the link is clicked. The "href" part is necessary as it won't work in IE without it.