td widths, not working?

It should be:

<td width="200">

or

<td style="width: 200px">

Note that if your cell contains some content that doesn't fit into the 200px (like somelongwordwithoutanyspaces), the cell will stretch nevertheless, unless your CSS contains table-layout: fixed for the table.

EDIT

As kristina childs noted on her answer, you should avoid both the width attribute and using inline CSS (with the style attribute). It's a good practice to separate style and structure as much as possible.

Width and/or height in tables are not standard anymore; as Ianzz says, they are deprecated. Instead the best way to do this is to have a block element inside your table cell that will hold the cell open to your desired size:

<table>
    <tr>
        <td valign="top">
            <div class="left_menu">
                <div class="menu_item">
                    <a href="#">Home</a>
                </div>
            </div>
        </td>
        <td valign="top" class="content">Content</td>
    </tr>
</table>

CSS

.content {
    width: 1000px;
}

.left_menu {
    background: none repeat scroll 0 0 #333333;
    border-radius: 5px 5px 5px 5px;
    font-family: Arial,Helvetica,sans-serif;
    font-size: 12px;
    font-weight: bold;
    padding: 5px;
    width: 200px;
}

.menu_item {
    background: none repeat scroll 0 0 #CCCCCC;
    border-bottom: 1px solid #999999;
    border-radius: 5px 5px 5px 5px;
    border-top: 1px solid #FFFFCC;
    cursor: pointer;
    padding: 5px;
}

This problem is quite easily solved using min-width and max-width within a css rule.

HTML

<table>
  <tr>
    <td class="name">Peter</td>
    <td class="hobby">Photography</td>
    <td class="comment">A long comment about something...</td>
  </td>
</table>

CSS

.name {
  max-width: 80px;
  min-width: 80px;
}

This will force the first column to be 80px wide. Usually I only use max-width without min-width to reign in text that is very occasionally too long from creating a table that has a super wide column that is mostly empty. The OP's question was about setting to a fixed width though, hence both rules together. On many browsers width:80px; in CSS is ignored for table columns. Setting the width within the HTML does work, but is not the way you should do things.

I would recommend using min and max width rules, and not set them the same but rather set a range. This way the table can do it's thing, but you can give it some hints on what to do with overly long content.

If I want to keep the text from wrapping and increasing the height of a row - but still make it possible for a user to see the full text, I use white-space: nowrap; on the main rule, then apply a hover rule that removes the width and nowrap rules so that the user can see the full content when they over their mouse over it. Something like this:

CSS

.name {
  max-width: 80px;
  white-space: nowrap;
  overflow: hidden;
}
.name:hover {
  max-width: none;
  white-space: normal;
  overflow:auto;
}

It just depends on exactly what you are trying to achieve. I hope this helps someone. PS As an aside, for iOS there is a fix for hover not working - see CSS Hover Not Working on iOS Safari and Chrome

 <table style="table-layout:fixed;">

This will force the styled width <td>. If the text overfills it, it will overlap the other <td> text. So try using media queries.