A placeholder is a Sitecore-specific tag within a layout or sublayout which allows the dynamic placement of components within it.
The main purpose of placeholders is to allow end users to have design flexibility, and minimizing the number of layouts you need. For example, rather than having a two or three column layout, you just have one layout with a placeholder and then drop a two-column or three-column sublayout into a content placeholder on a page. This has a number of advantages:
- Minimize layouts - generally speaking you can get down to having one layout for each site design and device. (Of course, microsites, etc. may require the creation of more - but Launch Sitecore is only built on one layout, despite the various page designs that are built in this layout through the use of placeholders)
- Placeholders can have settings designating which components can be added to each.
Note: Even if you use placeholders, it doesn't mean that users have to manually select components and sublayouts for each newly created page. You can use Standard Values to build a presentation automatically on an item, complete with components in placeholders, but then your users (if you give them permission) can override these defaults.