A linear website is one that is organized with a logical beginning, middle, and end, much as a printed book would be. Most websites are not designed to be linear, but it is a useful presentation for long-form content, such as manuals and online texts.
Linearity Versus Hypertext
A website is typically organized to be navigated using hypertext links between pages. For example, you might be reading the home page, then click on a link to take you to some other page on the site, then click on "FAQ" in the navigation bar to read the Frequently Asked Questions. There is no organized way to read this website from beginning to end. A linear website instead has a logical order, where you are expected to start at some beginning page and then work your way through to the end of a text or media series.
Other Types of Linearity
Most blogs inherit a limited type of linearity through the use of chronological organization of posts. There's no connection between a particular post and its previous one, but posts will be organized in the order they're written and presented with the newest one first. By clicking on an "older posts" button at the bottom of a list of posts, you can proceed linearly through a blog and read it entirely. However, if you use categories or tags to skip around sections of the blog, you are no longer reading it linearly.
Features of Linearity
A linearly organized website will typically present a table of contents, much like a book, and will have "next" and "previous" buttons to allow paging through the entire site. The "Next" button on the table of contents will typically take you to the first page of the website, then to the second page and so on. Many linear websites have an "Up" button to go back to the table of contents, in case you wish to stop reading a chapter and skip to the next one.
Exceptions to Linearity
Like a book, a linear website will have exceptions to its linear structure. Footnotes and endnotes are always hypertextual in nature; these might be included in linear text or on linked Web pages, so as not to break the flow of the main text. Likewise, a table of contents, index, and glossary are not meant to be read linearly, but are usually placed at the beginning and end of a linear Website respectively, much as they are in a book.