Primary databases are very comprehensive indexes to research literature and contain references to a wide range of original research.
The Cochrane Handbook mandates Medline and Embase (and Emcare if appropriate) as the key primary databases for your search. You should also search the Secondary databases.
Secondary databases only contain references to articles which have been assessed as having a high level of evidence using the hierarchy of evidence. They are usually much smaller than primary databases and many are available freely on the internet.
The Cochrane Handbook mandates the Cochrane Library as a key secondary database for your search.
Multidisciplinary databases can also be very useful to search, as these typically index different papers.