Sigma is a generic and open signature format that allows you to describe relevant log events straightforwardly. The rule format is flexible, easy to write, and applicable to any log file. The primary purpose of this project is to provide a structured form in which researchers or analysts can describe their once-developed detection methods and make them shareable with others.
Sigma is for log files is what Snort is for network traffic, and YARA is for files.
Source code and additional information may be found here: https://github.com/SigmaHQ/sigma