Logs are files on your disk (or data in memory) that apps write to when executing. Writing logfiles is enabled by the use of various logging frameworks in your application (for example, log4j for Java) and also the operating system's own system logs that track various system events. Databases, queuing systems, and pretty much every component of the app has logs. Anything your application writes to the standard error or output stream is usually captured in application logs.

Logging is arguably the most widely known resource for troubleshooting applications in any deployment. Logs are more often than not the first and most critical resource used to understand the status of the application when it crashes or starts behaving erratically. Logs are designed to collect information from disparate elements of your application and provide a time ordered view of how an application executes in real time and a log traces the events as they happen.