Every WordPress installation requires a theme. Themes control the header and footer on the page, as well as options such as a sidebar or footer for widgets and how blog posts are displayed. Themes also usually allow you to set some global styling options for your pages, such as fonts and colors.
The Beaver Builder plugin lets you easily create layouts in the content area of the page. That's the part of the page that is not controlled by a theme. For more information on the difference between the content area and theme areas of a page, see the primer article.
The Beaver Themer plugin is an add-on plugin that uses Beaver Builder to create layouts that you can apply anywhere on the page except the content area. For example, you could create a Themer layout for a header and use it to replace the standard theme header on all pages on your site or a subset.
Which of the three Beaver Builder products do you need?
The Beaver Builder plugin is the base product, and it's used for creating content layouts. It works with most themes and does not require the Beaver Builder Theme or Beaver Themer.
The Beaver Builder Theme is a well-coded framework theme that is tightly integrated with the Beaver Builder plugin. The Beaver Builder plugin plus Beaver Builder Theme give you enough flexibility that you'll probably never have to buy another theme.
Beaver Themer is the add-on to get if you want to do any of the following:
Create header or footer layouts that override your theme.
Create layouts for index, archive, search, and 404 pages that are dynamically generated by WordPress.
Create layouts to display single posts, which override the parts of the page that lie outside the content area.
With this type of layout, you use the WordPress editor to create the content that appears in the content area of the page.
Create layout "parts" that can be inserted above or below headers, footers, or the content area and specify where they will appear.
This layout type is ideal for inserting items like banners on a number of pages.
Beaver Themer requires the Beaver Builder plugin, but it doesn't require the Beaver Builder Theme, even though all three are designed to integrate well together. See the list of themes that are fully supported by Beaver Themer.