An "event layer" is the result of geometry being dynamically generated from a tabular source. A "feature layer" points to data whose geometry is already defined in a feature class. For example: you have a non-geometric table of xy coordinates. You use a tool to generate point geometry--features. The resultant layer, which is not yet stored in a geodatabase, is an "event layer." If you export the event layer and store it in a geodatabase as a feature class, viewing it in an mxd will be a "feature layer."