1. Free Wordpress Themes
When you download WordPress.org and set up your blog, you can use one of the free themes that comes with the application or find another free theme offered by a third-party designer. A Google search for "free WordPress themes" delivers more results than you can make sense of, but there are a number of sites that offer large varieties of free WordPress themes.
Here is what you need to know about free themes. They're great because they're free. However, many are not coded as well as themes that come with price tags and most don't come with any kind of support if you're having troubles. Also, since they're free, a lot of other blogs might be using the same theme. That means your blog might lack originality if you use a free theme.
2. Premium WordPress Themes
Premium WordPress themes come with a price tag. Typically, you can buy single-use licenses for a premium theme or a bundled package license, which is good for people with multiple sites, blog designers, and developers. Premium themes are usually priced between $50-$100, but some designers and theme clubs charge an annual membership fee to use all (or some) of the themes in their collections.
Here is what you need to know about premium WordPress themes. They are usually well-coded and some form of support is typically provided with theme purchase such as a free online support forum, free upgrades to new theme versions, and so on. Other providers require that you pay an additional fee for support. Email support is also possible.
3. Custom WordPress Themes
Custom WordPress themes are built from the ground up for your individual site. Don't be confused by a design that is simply a modification of an existing premium WordPress theme. Truly custom WordPress themes are coded specifically for the site they are used on. Custom WordPress theme design typically costs in the thousands of dollars, depending on the extent of your customization requirements and the breadth and depth of your site.
Here is what you need to know about custom WordPress themes. They can be expensive and if you don't know CSS, it could be difficult for you to make changes without hiring a designer to help you later. Keep in mind, the vast majority of bloggers get by just fine with a free or premium theme.