Wikipedia
SocialEngine is a software suite that allows creation of customizable online communities. It is sold in two flavours: as standalone software package called SocialEngine PHP, and as software as a service under the name SocialEngine Cloud.
SocialEngine PHP is a PHP-based white-label social networking service platform, that provides features similar to a social network on a user's website. Main features include administration of small-to-mid scale social networks, some customization abilities, unencrypted code, multilingual capability, and modular plugin/widget compatibility. There is a range of templates and add-ons available to extend the basic features already included in the SocialEngine core.
SocialEngine Cloud is a software as a service (SaaS), hosted version that facilitates interest graphs or communities of people with similar interests. The new cloud-based service lets anyone create a community website, including discussion communities similar to the microsharing style of Reddit, the photo curation style of Pinterest or activity stream style of Facebook depending on the theme.