Lightning App Builder is an integrated development environment for people who don't write code. Instead, the idea is to present a user with data and prebuilt off-the-shelf components that play well together. If you'd like to give it a spin, you can access the Lightning App Builder in any Summer '15 or newer org by clicking on Lightning App Builder under the Create submenu in Setup. The first step in creating a new Lightning App is to choose the template. Note the new column layouts, 2 and 3, that are available here:
Having selected a template, developers are taken to the App Builder Canvas, which looks like this:
The palette to the left shows a list of components available to the developer. Split into three segments, the first shows all of the built-in components, such as Filter List, recently used items, and Visualforce. After this, there are two groups of components. The first section display custom developed components and components available via AppExchange Packages...