In the HTML world, a Fieldset allows you to group thematically related fields on a form together with a visual line bordering them to denote their relationship. In addition, you can use the Legend tag to give the Fieldset a title. These tags serve two purposes in that they visually show the relationship between the items and they make the form more accessible. Here's an example:
Of course you could get more fancy with CSS, styling both the Fieldset and the Legend.
So...a pretty simple construct, but I think you can see it's usefulness. Great...that's fine for the web, but how do we do it in the Notes client? Actually, there are a whole bunch of ways to accomplish this using standard Notes dev techniques. The two I've found most useful for my purposes are to either create a graphic for my Fieldset and Legend so that they look exactly like I want them to OR use a...wait for it...wait for it...LAYER! :-)
Let's look at the graphic approach first. In your favorite graphic editing program, create the top of the Fieldset with the Legend. Make sure to match the color of the border and text with the visual theme of your form. You should get something that looks like this:
Now go to Domino Designer and create a table that will serve as your Fieldset. Paste the graphic into the first row of the table (you might need to merge cells first) or paste it directly above the table (I'll do that for this example). In order to make everything line up, first size the table so that it matches the width of your Fieldset graphic. Then, turn OFF the top border of the table. Finally, set the Picture properties of the Fieldset graphic so that the Text Wrap is "Don't wrap, align bottom". This will drop the graphic so it fits snugly against the table, appearing to complete the border and giving you a Fieldset with a Label.
If you like using layers like I do, then you'll find that they are an even easier way to add a simulated Fieldset to your form. To do this, you simply need to create a new layer and position it on the top line of your table so that it looks like a Fieldset. This allows for easier maintenance as well, since if you want to change the label, you simply type the new value in the layer. You can also resize it, change the color, etc.
There are other ways to approach this idea as well. Daniel Soares suggested creating an inner table within an outer table. The inner table would take care of the formatting for your field elements and the outer table, which would be a single cell, would contain a background image. This background image would be the entire Fieldset border. You could make some fancy effects with this one.
If I did a poor job explaining this or if you'd just like to explore it in more detail, I've put a Notes client database out there for you to download. There is a single form in the db that shows examples of each of these techniques and you can deconstruct them to see how they are put together.