The main log file in IBM Cognos 8 BI is cogserver.log file. This file can be located in the "../logs" directory. Depending on the tracing / logging levels information is logged in this file. In case you are getting any issue while running Cognos application, this is one of the starting points for you to start troubleshooting the issue.
For IBM Cognos Reportnet the file main log file is known as crnserver.log, this file is also located in the "../logs" directory for your reportnet installation.