Issues with sources: this is for tagging issues with sources; they are either inline tags, or message boxes for sections or entire pages. You could also leave a note on the talk page asking for a source, or move the material to the talk page and ask for a source there. There are also some templates for search links to find sources. Transcluded from Wikipedia:Template messages/Cleanup/Verifiability and sources.
Including third-party content: these are templates for referencing content for some specific sources: United States government, public domain and wikis.
Citations of specific sources: inline citation templates for a few specific sources.