Semantic MediaWiki (SMW) is a free extension of MediaWiki that helps to search, organise, tag, browse, evaluate, and share the wiki's content. While traditional wikis contain only texts which computers can neither understand nor evaluate, SMW adds semantic annotations that bring the power of the Semantic Web to the wiki.
Freebase is a database that has all kinds of data in it and an API. Because it's an open database, anyone can enter new data in Freebase. An example page in the Freebase db looks pretty similar to a Wikipedia page. When you enter new data, the app can make suggestions about content. The topics in Freebase are organized by type, and you can connect pages with links, semantic tagging. So in summary, Freebase is all about shared data and what you can do with it.