Online exhibits combine digital objects like photographs with descriptive narrative text.
Omeka comes in two versions: Omeka.net (hosted by Omeka) and Omeka.org (hosted by you). Omeka.net hosts your website for you, has subscription levels, and has pre-made themes and plugins. Omeka.org allows you to host your own website powered by Omeka. This gives you greater control over the appearance and functionality, but requires server space and manual maintenance and updating.
Plugins are specific features that can be added to Omeka. Here is a list of all the basic Omeka plugins.
Knightlabs has created a set of javascript libraries that work well with Omeka, here is a short list: