Widgets can be just anything: the title of the website, the title of an article, the content of the article, a comment form, a list of related posts, a rotating ad area, a menu item, a calendar, a list of tags, etc.
A collection owner may place any widget anywhere he wants on the skin of that collection, provided there is a widget container in that place of the skin. A typical widget container would be the Sidebar. But you will most likely also find a widget container for the Header containing widgets such as the "blog title" and the "blog tagline" (you could add a "logo widget", etc.). More recent versions of b2evolution tend to use Containers and Widgets for 100% of what is displayed in the Front-Office.
Each Collection can be configured with its own widgets. That means that even if 2 collections use the same skin, they may display very differently by placing different widgets in their containers and possibly also in a different order.
Widgets are basically anything that allows you to perform a specific function or access a specific service. Widgets can be an application, a web component, a little block of text or anything that can be displayed in certain areas (containers) in your…
Widget output can be cached in memory on the server using the BlockCache (requires a memory cache like APC to be installed).