I’m always asked this question, so thought it might be useful to provide a quick one liner for each.:
*_Prof – This is the is the profiles database. It contains all the information relating to peoples profiles, including those imported from the AD.
*_Site – This is the site database. By default it contains all the portal and team site information including documents and lists.
*_Serv – This is the service database. It contains alert definitions, notifications and the gatherer logs.
*_Config_db – This is the configuration database. This contains the actual definition of the “Farm” in which SharePoint Portal Server has been installed.