WordPress themes come in a LOT of varieties.  But, all of the varities can be narrowed down to a few basic families based on layout. 

• Single Column
• Two Column
• Three Column
• Magazine Layout

There are certainly variations on these options, but everything can be categorized in one of these groups. 

Single Column

This is the most basic... and difficult to find type of theme.  Blog posts simple run down the web page, from top to bottom.  there are no sidebars with distracting links or other content.  In some cases, links and other content could be placed at the bottom of the web page... or possibly even the top.  Of course, 'pages' could be deployed with content, but they wouldn't be on the main landing zone. 

Two Column

Now we add a column along the side that we can toss in contact info, pictures, links and widgets.  These are among the most popular layouts.  Here is an example.  Of course, some themes will still allow content at the top and/or bottom, as well as additional pages. 

Three Column

I have a playsite that I am working on, and I have deployed a three column theme for it.  Basically, we are adding a second sidebar for content.  Both sidebars can be on the same side or they can be on different sides.  These are also pretty popular layouts.  The advantage is that there can be more content introduced on the 'Landing Area'.  The drawback is that the content area is narrower. There are also some Four Column layouts, but they are pretty rare and for many people they are quite crowded...  You are better of going to a...

Magazine Layout

These are seriously flexible.  Here is a development site I am doing for my wife.  This is a magazine style layout.  Instead of the posts being front and center, featured content is more controlled.  Using a magazine layout, one can take WordPress and turn it into a more full featured website instead of "just a blog".  The presentation is more tightly controlled, and so the look isn't really that of a blog on the front page. 

There are also hybrids.  I run a theme on LaneBailey.com that can be a magazine type portal or traditional blog.  As a traditional blog, I can deploy 'user positions' at places all over the blog, allowing it to still have a magazine flavor, but function as a blog.  During the recent VAR Blog Brawl, I put a notice asking for votes on my blog above the content between the sidebars.  I also have the ability to move the sidebars from side to side... as do the readers (just below the header on the right side). 

Glossary...

page - static content for the site...  also can note specific views of the site, like the landing page, archives, tagged content, which aren't static. 

widget - I code snippet that does something, like the one that displays posts from my other blogs on the sidebar.

landing area - The first view of a site when someone clicks a link or types in the address.

user positions - Sidebars, footers, and areas where the blog posts aren't, but there is content.  The best way to show you is to go to my joomla site, GarageHomesUSA.com.  Everything you see on the page, except the three stories smack dab in the middle of the page, is in a user position.  Through the back-end of the site I can determine exactly which position in which I want each item to show.  You need to view that site in FireFox or Internet Explorer 8 while I trace down whatever is breaking the site in IE7 and under. 

footer - The bottom of the web page.

header - The top of the web page

***Update***

I realized that I wasn't very clear.  This discussion pertains to WordPress in the self-hosted variety... WordPress.org, not WordPress.com.