Wordpress templates are the easiest way to get professional quality web design on a very limited budget. Design companies, independent designers and enthusiastic hobiests produce them and provide them for your use for free as advertisement or to build their profile. Many request that you keep a link to them in your footer, but that's not always necessary. More importantly, most of them are very top-knotch designs with lots of great functionality that makes the task of creating a blog or website that looks great very easy.
It used to be that you needed to download template files, unzip them and then FTP them up to your Wordpress template directory before you could activate them. Ah, the good old days! Most templates are now easy to install directly from your wordpress admin area. Simply go to "appearance" and search for the them you want to install (they'll all be accessed from the Wordpress.org site) then click "install" and "activate". Yep. That's it!
If you find a template that's not part of the auto-install scene, you can still use it but you'll be best to follow the directions at wordpress.org as to exactly how to upload and install a new template.
These are my favorites, not an all-time 'best of' list so here's a bit about my choices. Everyone has their own favorite styles. I personally love a right-hand column most of the time with a large center space. I don't usually like adjustable width templates, although many people do. I DO love a template with easy-to-change headers (prefferably one that will adjust any picture I uplaod to the right dimensions). I like standards in in web design that says to use one or two colors in varring shades with on accent color. Finally, I'm always partial to magazine-style themes because I like how it offers more information 'above the fold' for first time visitors.
Ok, that said, here's my top 5 favorite FREE wordpress templates:
THE MORNING AFTER - by Woo Themes is an amazing free magazine theme and one of my favorite wordpress templates ever. Because it's magazine style, expect some of a learning curve as you figure out how to use it to it's full potential. I've never run a magazine style blog with this theme, but literally ALL my favorite blogs that I read regularaly use this theme.
The ARAS THEME - by Melvin Lee is fun to use for the fact that it has just SO many different customization options. This is one of my favorite wordpress templates to use when putting together a site that will need basic customization but not a whole new template.
The News Magazine 360 is another fun magazine style blog theme. I especially like how this one has support tools for customizing the CSS to make it truly unique.
Shades is another great free theme, but it doesn't have it's own home page. I love how clean and simple it is to use. One of my favorite 'traditional' blog style themes.
Finally, Atahualpa is what's replaced my old favorite theme, Alkivia, which has been discontinued. I love how it's got all the functions on the back end to help you truly create something unique for your site. No programming knowledge needed. This is one of the many new wordpress templates that helps you create a unique header with back-end cropping tools.
To modify the files, you can either edit them directly in the back-end of your Wordpress Template manager, or you can play it safe and download all your files and mess around with them in your WISIWIG program. Then, when you've got all the CSS files fixed up how you want them, re-upload them again. Simple, right? Yeah, go have fun with that.
The best thing about templates are that most of them are free. Even if you have to pay something basic for your personalized template, the CSS style of Wordpress means that these templates are usually cheaper than other website development projects. If you can find one that works well for the type of website you're building, then all you need to focus on is creating good content for your pages.