Slack is a messaging app for teams that allows organization of conversations into channels, private channels, and direct messages. It integrates with other apps like Skype for video calls and allows sharing of files. Slack was created in 2013 and has grown rapidly in valuation. The document discusses key Slack features like notifications, calls, drag and drop file sharing, and powerful integrations that bring different systems into one searchable place for teams to communicate and work together. The conclusion encourages signing up for a Slack account to access these collaboration capabilities.