A theme is responsible for the look of your website. By default, there are three themes you can choose. You can see them when you navigate to Dashboard | Pages and Themes, as shown in the following screenshot:
A theme isn't restricted to a certain layout structure like a portal. concrete5 themes can be built using any HTML and CSS code you want. You can even create your JavaScript files on the fly. You therefore find lots of different layouts and whatever you'd like to build; there are pretty much no limits.
You can activate a theme by clicking on Activate. Confirm the activation on the next screen and your website changes its layout immediately. If you're working on an active site, you might want to hit Preview first to see how your page is going to look with the new theme.