This page in a nutshell: A navigation template is a template that links between existing articles belonging to the same topic on English Wikipedia. There are two types of navigation templates: navigation boxes, or navboxes, and sidebars.
A navigation template is a grouping of links used in multiple related articles to facilitate navigation between those articles. Editing of a navigation template is done in a central place, the template page.
There are two main varieties of navigation template: navigation boxes (or navboxes), designed to sit at the very bottom of articles, and sidebars, designed to sit at the side of the article text. The two are complementary and either or both may be appropriate in different situations.
The usual way to create navigation templates is to use the ((navbox)) or ((sidebar)) master templates. This simplifies the process of creating a functional and consistent template.
Do not rely solely on navboxes for links to articles highly relevant to a particular article. Navboxes are not displayed on the mobile website for Wikipedia, which accounts for around half of readers. See Phabricator ticket T124168 for progress on the mobile issue.
Sidebars sit alongside content, in the same manner as infoboxes, and are predominantly laid out vertically. They are created using the ((sidebar)) template. An example sidebar is shown to the right:
The two types are used interchangeably, and either or both may be appropriate in different circumstances. The primary differences between the two are:
Navboxes are laid out horizontally, and so work best for longer lists of links in a small number of sub-categories. As they are placed at the very bottom of articles, they are better for broader lists of links than would be appropriate in a sidebar. Articles often have more than one navbox and content may overlap to a degree: nevertheless, not everything needs a navbox, so navbox templates should only be created when they would be genuinely useful as navigational tools.
Sidebars are laid out predominantly vertically, and are placed relatively prominently in the body of articles alongside the text. This makes them useful for smaller amounts of directly relevant links. Tangential information should be kept out of sidebars. Few articles have more than one sidebar.
The style of any navigation template depends on its articles, how they are most intuitively presented, and previously established convention.
Navigation templates provide navigation within Wikipedia
They are intended to link articles to each other. That is, every article listed on a particular navigation template generally has the template placed on its page.
The goal is not to cram as many related articles as possible into one space. Ask yourself, does this help the reader in reading up on related topics? Take any two articles in the template. Would a reader really want to go from A to B?
They should be kept small in size as a large template has limited navigation value. For navigating among many articles, consider:
Split them into multiple, smaller templates on each sub-topic. For example, ((EMD diesels)) lists all models of diesel locomotives built by one manufacturer, but is too large to be transcluded on each of their articles. Instead, the individual sections of ((EMD diesels)) were split out into their own templates: ((EMD GPs)), ((EMD SDs)), etc.
Red links should normally be avoided unless they are very likely to be developed into articles. Red links can be retained in navigation templates that represent a well-defined and complete set of data (geographic divisions, annual events, filmographies, etc.), where deleting red links would leave an incomplete and misleading result. Even then, editors are encouraged to write the article first.
Likewise, redirects should also normally be avoided for the reasons outlined in WP:NAVNOREDIRECT. The general exception to this is when the redirect represents a distinct sub-topic of an article rather than simply being an alternative name. For example, ((MSPA)) links to Jailbreak (webcomic) despite it not having its own page for the sake of completeness.
Unlinked text should be avoided.
Note: In navigation boxes about musical ensembles, it may be appropriate to list all of the members of the ensemble, to avoid the perception that the ensemble is a solo act, provided that at least one member of the ensemble is notable.
Navigation templates provide navigation between related articles
If the articles are not established as related by reliable sources in the actual articles, then it is probably not a good idea to interlink them.
For complex topics in science, technology, history, etc., a navigation box can provide a comprehensive introduction to a topic. For example, ((Wind power)) links to subsidiary and supporting topics that provide background and context necessary for understanding the main Wind power article. While the main Wind power article already contains inline links to the subsidiary articles, the subsidiary articles themselves are smaller and their prose may not place them into the overall context with each other. Editors who work on the subsidiary articles in isolation may be unaware of this context. The navigation template provides an easy way for the subsidiary articles, even when they begin as stubs, to instantly inherit the conceptual structure of the main article.
Navigation templates are not arbitrarily decorative
Per MOS:DECOR, images are rarely appropriate in navboxes. Just like colors and styles, they should have a justification to appear. Specifically, there should be no national or regional flags or coats of arms. A rare example of an appropriate image is this: a map shows (in green) the location of a region within the state of Kazakhstan, and this is consistently implemented for all state's regions.
compactness of the template compared to a standard list or table, in the case of many links,
if the most immediately related links are kept under "See also", the reader has a better idea of scope,
less directly related links are out of the way or in some cases hidden by default,
ease of maintenance in updating the template as articles get created or deleted,
aesthetically pleasing appearance to many users,
new articles in a subject area immediately gain the basic link structure of existing related articles, eliminating the need for many editors to individually build up their own links and rewrite background material.
when a new article or an older article that was orphaned is placed in a navbox, the page instantly has a large number of links to it
In certain cases, there are alternatives preferable over the creation of a new navigation template.
For a series of articles whose only shared characteristic is that they hold the same position or title, such as peerage or world champion sporting titles, consider using ((succession box)). Variant templates for persons who have held several notable offices are discussed at Template talk:Succession box.
For relating articles across different categories in a logical sequence, use a succession template.
There are limits to the number of templates an article may have. When a page exceeds this limit it may look fine in preview but, after the edit is saved, one or more footer navboxes display as wikilinks to the now excess navboxes (for example, displaying a link to "Template:Navbox" rather than the Navbox template itself). Solutions for this problem include (a) removing a template, and (b) setting up the footer navboxes so the least important one becomes the "extra" navbox (the one the reader will have to navigate to a separate page to in order to view).