If you have trouble getting your head around Loggers (ex Categories) and Appenders, perhaps it will help to think of them as sources and sinks with filtering methods available on each.
In log4cxx/log4j context Logger objects are the sources of logging data. Older examples or documentation may refer to the Category object. It is essentially the same thing. Logger has replaced Category.
You can filter Logger statements by changing the log level for a given logger. This turns on or off various source points for a given logger if it uses various log levels. You can override all the Loggers levels by setting the Threshold of the LoggerRepository object. (It is ALL by default)