Slack is a collaboration tool that replaces email with direct messaging and group chats, allowing users to seamlessly attach documents from services like Google Docs. Originally built as an internal tool, it launched publicly in 2014 and has since grown to over 2.3 million daily active users, with $64 million in annual recurring revenue and a $3.8 billion valuation after raising $500 million over 7 funding rounds. Slack's integration with other services and search capabilities within chats and documents give it a competitive advantage over tools like HipChat and Yammer.