Splunk is a tool that allows users to search through log files and machine data from servers, databases, applications and other systems to troubleshoot issues and gain insights. The document provides examples of how Splunk was used to resolve a website outage by searching logs, track increased online traffic due to a celebrity tweet, and improve an online shopping experience. It also discusses how Splunk works, the types of machine data that can be analyzed, and how operational intelligence benefits organizations.