Say you've got a 2x2 table with usernames + post counts.

<table>
 <tr>
  <td>Robin</td><td>300</td>
 </tr>
 <tr>
  <td>Ahmed</td><td>400</td>
 </tr>
</table>

You'd want to add headers to show up what the columns mean. To do this you'd use th elements (table header):

<table>
 <tr>
  <th>Username</th><th>Post count</th>
 </tr>
 <tr>
  <td>Robin</td><td>300</td>
 </tr>
 <tr>
  <td>Ahmed</td><td>400</td>
 </tr>
</table>

Of course, if you want to go more accessible still you'd add some more features. For example:

<table summary="A table containing a list of users and their respective post counts">
 <caption>User details</caption>
 <thead>
 <tr>
  <th scope="col">Username</th><th scope="col">Post count</th>
 </tr>
 </thead>
 <tbody>
 <tr>
  <td>Robin</td><td>300</td>
 </tr>
 <tr>
  <td>Ahmed</td><td>400</td>
 </tr>
 </tbody>
</table>

Say you have a table of poples names, sex, and ages.

The headers would therefore be Name, Sex and Age

So, you might markup your table like so:

<table>
<caption>Peoples Data</caption>

<tr><th>Name</th><th>Sex</th><th>Age</th></tr>

<tr><td>Joe</td><td>Male</td><td>21</td></tr>

etc etc

</table>

HTH

Ben

added: Alternatively, see robins much more thorough post above :)

That's fantastic! Thank you!