tizag.com is using tables for the layout. you can use the same principle using only CSS if you want.
To do that you need the <div id="">and the </div> in your primary html page. if you are wanting the solid border then in your css you would have somthing like this
border-top: solid 1px #000000
border-left: solid 1px #000000;
border-right: solid 1px #000000;
border-bottom: solid 1px #000000
If you are wanting rounded corners then you need to make a image for the corner using a program like paint shop pro, illustrator, fireworks etc... Then save it as a gif for example top_left_corner.gif, then put this code in the html of your page.
In the code below I am using a curled corner only for the top of my content area. but this should give a little help on that.
<img alt="" src="top_left_corner.gif" height="6" width="6" id="tl" /> <img alt="" src="top_right_corner.gif" height="6" width="6" id="tr" />
<p>blah blah blah</p>
Here is the CSS code for the image part.
This is only an example, don't feel bad about experimenting to get the look you want. With Dreamweaver you have quite an arsenal of valuable tools to use. If you aren't all that familiar with dreamweaver then I would recommend reading some tutorials on it. You can find them at the macromedia web site
as well as pixel2life.com
and others are available too, do a google for them there are tons of them out there.