Containments are applets, that contain other applets. The desktop panel, for example, is a containment. It allows the developer to group sets of applets together and provide a common look & feel for the contained applets. An applets look is of course mainly determined through themes, but with containments a developer can impose certain size constraints on applets, he can pin down how items are laid out inside the containment, he configures the background of a containment and defines a context menu for it. Notable containments so far (as of 03/2008) are Panel and DefaultDesktop.
"... everything that holds a group of desktop widgets is a Containment." Also the rest of Aaron's blog post (see Sources below)