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.