A layout is a piece of layout code (usually a fragment of HTML) whose job it is to lay out part of an Actinic store page.
The section on 'Overall Page Layouts' (see Editing An Overall Page Layout Within Actinic) talks about the layout that controls the elements which do not change from page to page. The 'Overall Page Layout' is one layout, and a very important one, but there are hundreds of other layouts for controlling the appearance of all the little elements that make up an Actinic store page.
Every element in your store design has a layout that controls how it looks - there is a layout for the quick search bar, a layout for each icon in the store navigation bar , a layout for your products, a layout for the shopping cart grid etc. etc.
It sounds like a lot to understand, but don't worry! There are some simple ways you can change the design of an Actinic page, and make it look exactly how you need it.
Every kind of item in the 'content tree' (e.g. products, attributes, components etc.) has a range of alternative layouts to choose from - which you can select from the 'Layout' tab. This means you can change the appearance of the items in your store, without having to go anywhere near any HTML code!
This is discussed in more depth in Choosing Alternative Layouts in Actinic.
There are hundreds of different layouts in Actinic for all the different elements that make up a complete Actinic page design. However, you don't need to remember which layout does what, all you need to do is click on an item in the preview in the 'Design' tab that you want to change the look and feel of, and you will immediately be shown the layout to edit.
This is discussed in Editing Layouts.
You can also see the full range of layouts in Actinic in the 'Library'. See Using The Library.
Some layouts are laid out in lists - like the product list or the section link list. Read more about lists in Content List Overview.
Layouts are usually inserted into other layouts. An example of this is the product image layout, which is inserted into the product layouts. When layouts are inserted into other layouts, a thing called a 'layout selector' is used to make the insertion.
An example of a layout selector is 'BestSellerList', which inserts a list of best sellers into the page.
A layout selector is a bit like a variable, but instead of inserting a piece of text into the design (like a product name) it inserts a layout into the design. And just like you can set a value for a variable when you are editing your products and sections etc., you can also set the values of some of the layout selectors in Actinic. This is all explained in Choosing Alternative Layouts in Actinic.