CONTENIDO has a lot of different logs on the filesystem and the in the database that contain information about what CONTENIDO is doing and about what its users are doing.
The logs, which are saved on the filesystem all reside in /data/logs/. The logs that are in the database are accessible via the Logs backend page (Administration > Logs).
This file contains informations about errors that happened during the setup or during an upgrade. If you are experiencing problems after the installation of CONTENIDO or after an upgrade, you can take a look in here to find out what went wrong.
Exceptions that occur in CONTENIDO will be logged here. If you are experiencing problems and nothing shows up in the errorlog.txt, it is worth to look in here since some parts of the CONTENIDO Core use exceptions to relay errors.
Everything that goes wrong in CONTENIDO will be logged here. PHP errors will also be redirected to this file and won't show up in the normal PHP log. If you have any issues in CONTENIDO this is the first place to look at.
The CONTENIDO Core produces debug messages to more easily find bugs in the program. Usually these messages aren't shown anywhere, but you can turn debug messages on in the backend system configuration. If you choose to save debug messages to a file, this file is used.
You can find out more about backend debugging here.
Every action that a user takes is logged here. This is useful to figure out if the users are doing what they are supposed to do and to quickly see if an account got compromised.
If you choose to turn it on in the system configuration (Administration > System > System Configuration), all mails that are sent by CONTENIDO will be logged. You can see them here and retrieve information about them (for example to check if they got sent or not).