If we accept the "garbage in, garbage out" axiom as the basis for a metaphor, then openacs is like a recycling and waste management center, whereas a wiki is like a collection of garbage. And a recycling center is nothing without garbage! =)
That is, a wiki collects free-form input on 1 or more pages, and depends on users to moderate the presentation of it. OpenACS has a wiki-like package[1] called "Edit this page".
OpenACS is a set of high-level "plug-in" applications called "packages" to filter and process direct user input in other, more controlled ways, such as forums, shared and individually controlled calendars etc. Additionally, a core "kernel" API and package components are available to make other applications and indirectly process input using "services".
For a more complete overview, see the OpenACS documenation about "What is OpenACS?"[2]
1. https://openacs.org/projects/openacs/packages/
2. https://openacs.org/doc/openacs-5-1/openacs-overview.html
