An OpenACS Package is a collection of templates, Tcl and SQL library procedures, and datamodel definitions that cooperate to provide a single application or service.

An application is a package that directly provides functionality that solves a single set of user needs. (vague but the best I can come up with at the moment!)

A service is a package that is not meant to be visible to the user in a standalone-fashion, but rather provides services to other packages.

Examples include the forums and bugtracker applications, and the notifications service.

Packages should not provide a suite of applications, but should be written atomically.

An OpenACS website is a collection of package instantiations mounted in a single OpenACS instance. Most OpenACS developers make at least part of their living creating custom websites for clients. Such websites generally are built from standard and customized versions of existing OpenACS packages, along with custom packages built specifically for the client.

A vertical application is a pre-configured set of OpenACS packages that build a website containing pre-determined packages and a pre-determined look-and-feel. Currently .LRN is the only supported OpenACS vertical application.